#n5g

"Teacher watches TV every day."

Teacher (subject) + watches (verb) + TV (direct object) + every day (time period)

As you can tell, TV is the object which is being modified by the subject's action of watching. Just like the modifier in the last sentence, "every day" is just an extra detail which does not affect the main structure of the sentence.

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"What did Mary do on the weekend?"

What (direct object) + Mary (subject) + do (verb) + on the weekend (time)

Even though this is a question, it works the same way. "what" is what was "done" and so therefore "what" is the direct object.

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Note that all of these actions are active (transitive), not passive. You cannot use を with intransitive, or passive verbs, because they do not have a direct object. For example, "I broke the toy" and "the toy broke". In the latter sentence there is no subject to directly affect the object. More on transitive vs. intransitive(x).

Now let's try applying this in Japanese. 

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Usage:

"I read the textbook." -> わたしはきょうかしょをよみます。

Textbook, or きょうかしょ, is the direct object as we learned before so here we attach を to the end of it to indicate that it is what is being read.

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"I ate a lot of cake." -> わたしはたくさんケーキをたべました。 

Again, we attach を to the end of cake or ケーキ since it is what is being eaten. 

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"Teacher watches TV everyday." -> せんせいはまいにちテレビをみます。

The TV or テレビ is being watched by the subject and is the object marked by を.

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"What did Mary do on the weekend?" -> メアリーさんはしゅうまつになにをしましたか。

Looking at this as a statement, Mary did "what" on the weekend, so what or なに is marked by を.

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Passive vs. Active:

"The car stopped." -> くるまがとまりました。

Since there is no subject actively doing the action, "car" or くるま becomes the subject and is marked by the subject marker が instead of を.

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"I stopped(parked) the car." -> わたしはくるまをとめました。

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Conjugation Rules: 

る-verbs (drop る and add て) 

ex. 食べる（たべる）-> 食べて（たべて）

ex. 見る（みる）-> 見て（みて)

Irregular verbs:

する -> して

来る（くる) -> 来て（きて）

This seems pretty simple so far, right? Unfortunately, it gets a lot more confusing when it comes to う-verbs. Depending on what ending う-verbs have, they conjugate differently. If it ends with:

う, つ, or る (drop final letter and add って)

ex. 買う（かう）-> 買って（かって) 

ex. 待つ（まつ）-> 待って（まって）

ex. 取る（とる) -> 取って（とって）

む, ぶ, or ぬ (drop final letter and add んで)

ex. 飲む（のむ）-> 飲んで（のんで）

ex. 遊ぶ（あそぶ）-> 遊んで（あそんで）

ex. 死ぬ（しぬ) -> 死んで（しんで）

く (drop く and add いて)*

ex. 書く（かく) -> 書いて（かいて)

ぐ (drop ぐ and add いで)

ex. 泳ぐ（およぐ) -> 泳いで（およいで)

す (drop す and add して)

ex. 話す（はなす) -> 話して（はなして)

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*Note: 行く（いく) is an exception to the rule and becomes 行って (いって).