#preterite

ser and ir share the same conjugation in preterite [fui, fuiste, fue, fueron, fuimos, fuisteis]

Monosyllabic conjugations do not take accent marks i.e. fui, vi, di, fue, vio, guio, crio, crie, guie, dio etc. Outside of your standard monosyllabic verbs [ser, ir, ver, dar] you see this most with guiar "to guide" and criar "to raise (children)" / "to be brought up" -> though occasionally you see an irregular one like reír "to laugh" in preterite is considered 2 syllables but io is considered one syllable [like yo], so preterite reír preserves its accent mark with reí and reíste etc but then the 3rd person rio has no accent mark; it's linguistics and hard to explain and I wouldn't bring it up but you will see se rio "they laughed", or se rio (de alguien) "they laughed at someone / they made fun of someone"

Stem-changing verbs tend to only have irregularities in -ir verbs in 3rd person [as an example entender in present tense is I->ie, entiende etc... in preterite it's entendió; but seguir in preterite is sigue then to siguió] -> In most cases these irregular changes are for -ir verbs -> In most cases the -ir verbs that have irregular changes are E->I and E->IE; not counting verbs that are wholly irregular like venir and decir and querer -> The same syllable that had the change in present tense will change in preterite, just with an I -> Example: preferir is prefiere in present -> prefirió; then pedir is pide -> pidió; so while preferir is E->IE and pedir is E->I they share the same conjugation pattern as stem-changing -ir verbs -> The main exceptions to this are dormir and morir which are the O->UE verbs that end in -ir; poder is wholly irregular........ in other words in preterite it seems normal until 3rd person where the O changes to a U; durmió and murió, then durmieron and murieron

The other rules to be aware of are those regarding verbs that have -eer, -aer, -uir etc. and then verbs ending in -car, -gar, -zar, -guar which are changes based on sound

In essence let me explain with leer ...in preterite the rule is to add -ió and -ieron to 3rd person. This creates a bit of an odd case where you would be making "leió" and "leieron" which is an unnecessary seemingly extra syllable

The issue there is that EI is a diphthong [two vowel sounds pronounced as one] and so is IÓ... in other words rey "king" is one syllable [EI sound], and reina "queen" is two syllables [EI sound]; similarly, comío is two syllables with com-ío

With leer and verbs like it you find yourself combining this sounds into a kind of triphthong [three vowel sounds pronounced as one]. For the sake of brevity and pronunciation it has to change

The verb conjugations for verbs ending in -eer, -eir, -aer, -uir are usually based on preserving the vowel sounds; either adding Y to keep it from being too odd [like how "leió" would be], or for preserving that initial vowel sound by adding an accent mark [like leíste, creíste, oíste etc]

leer

yo -> leí tú -> leíste él/ella/usted -> leyó ellos/ellas/ustedes -> leyeron nosotros -> leímos vosotros -> leísteis

This also applies to -aer [caer -> cayó/cayeron], -uir [contribuir -> contribuyó/contribuyeron], and a bit of an oddity like oír "to hear" [oír -> oyó/oyeron]

The only verb that I can think of that's an exception to this particular one is traer and verbs related to it because for some reason it goes to traje, trajiste, trajo, trajeron, trajimos, trajisteis - which is unusual

I say it's unusual because that particular conjugation pattern is more common for -cir and verbs; though linguistically traer is related to "traction", "attract", "detract", "retract" so that's probably why

But for reference decir goes to dije, dijiste, dijo, dijeron, dijimos, dijisteis which is usually counted as its own irregular verb...

Or conducir or introducir follow that pattern; conduje / introduje etc.

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The next big rule based on sounds is the -car, -gar, -zar, and -guar verbs. These are based on phonetics. They do follow regular rules, but in all of these cases it's the yo form that's the issue

-car for example has a hard C/K sound; but CE is a soft C/S sound

A verb like colocar "to place" wants you to preserve that hard -car sound, that's why it turns to coloqué in yo because QU has that hard C/K sound

Similarly pagar if you wrote GE it would have a soft H sound [like la gelatina]... to keep that hard G sound you had the GU to it as pagué so that it's pronounced closer to guerra "war" would be

-zar has a similar problem but a bit linguistically different; empezar has the Z/S sound but now you want é... but ZE isn't a letter combination you often see in Spanish (neither is ZI for that matter)... as in "zero" is cero ... so to get that sound you substitute the ZE for CE; empecé

And -guar is not a verb ending you see often so it's not always in textbooks, so this might take some explanation

averiguar "to find out" has a GUA sound, which would be like GWA in English; that W sound is there is the point. If you add the E, you get that same problem in reverse now; GUE omits the W sound to a pure G sound

How to preserve the W sound? It becomes averigüé with the umlaut ü ; the umlauts [called "dieresis" in linguistics] aren't common, but they do show up over U every so often. In all cases it's to change the U [ooh] sound to more of a W sound....... The not-linguistic reasoning is that ü is like half the sound or you half open your mouth to say it, so if you interrupt the U sound in Spanish it comes out a bit like a W

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ALSO - verbs ending in LLIR/LLER and ÑIR/ÑER are a bit irregular

The LL and Ñ are almost considered vowels for preterite conjugation. It's unlikely you'll see LLIR/LLER verbs as they're pretty rare, but this is how it would work:

bullir "to bubble/bustle" [uncommon word, you may see el bullicio as "hustle and bustle" as a noun, but it meant "bubbling" or agitation/activity]

...You're unlikely to see bullir but if you do it's usually in reference to "reach a boiling point"

engullir = to engulf / to wolf down food

bullir = to bubble, to reach a boiling point [more common is hervir "to boil"]

zambullir(se) = to dive [as in the sport or something involving a diving board etc; bucear is "to dive" like scuba diving]

yo -> bullí tú -> bulliste él/ella/usted -> bulló ellos/ellas/ustedes -> bulleron nosotros -> bullimos vosotros -> bullisteis

Keep in mind LL has a Y/I sort of sound and you'll understand the reasoning behind bulló and bulleron... because it comes out a bit like "buyo" and "buyeron", you don't need an additional I

While it's regular, you would expect -ió or -ieron, but because LL fulfills the function of the I sound, it can skip it and just add the -ó/-eron

More likely you will see a verb ending in ÑIR though

gruñir = to growl, to grumble

tañer = to ring (a bell) [sometimes it means "to play an instrument" but you usually use tocar for that - this I've heard specifically for ringing bells]

teñir = to dye / to tinge

bruñir = to polish [pulir is more common; bruñir is "to burnish", so more literary but can be common in some contexts]

ceñir = to cinch / to cling to, to stick to [usually clothes; like a "girdle" is el ceñidor but this is when fabric is more constricting or tighter as opposed to being loose, but ajustado/a "tight" and ajustar "to adjust/tighten" is the more common word]

ceñir(se) = to surround [often said of places, like la niebla (se) ciñó la ciudad is "fog encircled the city" or "fog clung to the city" that sort of feeling, or la muralla ciño la ciudad "the battlement encircled the city"...... ceñir(se) can have other meanings but it's usually attachment or adherence in its meaning]

teñir and gruñir are the ones you're most likely to see

Like teñir(se) el pelo is "to dye one's hair"

yo -> teñí tú -> teñiste él/ella/usted -> tiñó ellos/ellas/ustedes -> tiñeron nosotros -> teñimos vosotros -> teñisteis

Note: teñir is E->I stem-changing, that's why it's tiñó/tiñeron

But the idea is the same that Ñ has a Y/I sound to it, so 3rd person can omit the additional I

This also works with a regular -ir verb; gruñó/gruñeron for "they growled" is quite common to see in stories... both for monsters and to indicate a tone of voice

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This has nothing to do with what you asked, but just keep in mind that Spanish (in general) goes out of its way to help you spell things in a uniform way - so the sounds make sense, but the spelling might confuse you until you learn Spanish phonemes [sound particles] or in other words, how words sound in Spanish with certain combinations of vowels and consonants

Big Random Linguistic-y Tangent

You sometimes see ü more in Spanish approximations of English words that had a W. Particularly peninsular Spanish [Spain] hates Anglicisms [English words] and they used to change names much more often, they go out of their way sometimes to write it phonetically the way a Spanish-speaker would understand it. This usually involves GU or the ü

Primarily you're going to see this with W sounds, K sounds, and double letter English words

As an example guau is "wow"; it's that exact sound [it's also the sound of a dog barking so guau guau is like "bow-wow", or "woof"]

Another one is "whisky" is sometimes written as güisqui - güi for "WH", then squi for the "skee" sound

The name Wendy is sometimes written as Guendi or Güendi (just for fun)

And older names particularly names from the British Isles would get translated all the time - Guinevere in literature is Ginebra [also the word for "gin", and "Geneva" in Switzerland]

...Also speaking of Switzerland, that would be (la) Suiza because of the W sound, or suizo/a for "Swiss"

They don't do it anymore I don't think but my textbook wrote el quiosco for "kiosk" because they really did not want the K to be used too much

But I also see guiverno for "wyvern" in fantasy, grogui for "groggy" [like sleepiness or dazed just like English]

A funny one is that huevón "dumbass" [lit. "big egg/testicle"] would sometimes be written as güevón then Chile decided to write it as weón