3 raccoons in a trench coat

I've seen people assume that the way Rocky speaks is a quirk of Eridian just being a simple language. While yes, that could be true to an extent, both the film and especially the book make it very clear that Rocky only speaks in stilted, broken sentences because of the language barrier. By the end, he speaks perfectly clearly in complete sentences with humour and sarcasm and natural sounding exclamations. Grace is at that point fluent and understands all the nuances.

This leads to the first hurdle of Rocky's in communicating with Grace in the beginning: Grace is stupid. He's not but to someone from a species that has the auditory equivalent of photographic memory (I don't think this is mentioned in the film but it is in the book), Grace's lack of such makes it difficult to share a language quickly. Grace needs spreadsheets, audio synthesis software, coding, text-to-speech in the film because he can only "hear" (read: see) in one direction at a time. He's functionally disabled to Rocky. Rocky therefore accomodates him which is very sweet. Here's a list of ways in which Rocky is most likely altering his language use for Grace's benefit:

He speaks clearly. It's mentioned in the book that at one point Grace's translation software (which he programmed himself for all the only-film fans) fails to translate a sentence of Rocky's. Grace is confused and confirms that the software is fully functioning. He realises that the issue is the pitch. Rocky's voice dropped an octave. When he shifts it back into the range the software recognises, it's perfectly translated. This means Rocky must constantly be keeping his inflection and pitch extremely steady and repetitive so that Grace's computer can keep up. There's an argument to be made that Eridian might just be like that, but knowing what we know about it (pitch change for emotion, it being a highly auditorily complex language made of musical chords, most likely created from at least five sets of whatever equivalent of vocal chords Eridians have, etc.), I find it highly unlikely that Eridian would be so even. It's much more likely that super genius Rocky, despite never having seen a computer before, quickly realised that it couldn't keep up with those variations without serious programming that Grace isn't good enough at coding to do or manually entering every variation of a word. It's simpler to just speak evenly. And clearly, any mumbling would be a nightmare for the computer to pick apart.

He uses words he knows Grace knows. Obviously he sometimes needs to use words Grace is unfamiliar with and then they're added to the translator, but have you ever tried to speak your native language to a novice speaker and limit your language to the words they know? It's way harder than it sounds. You naturally fall into expressions and phrases and words etc. that you use with other native speakers, and you often have to repeat yourself with altered versions of the original sentences because you accidentally used words the novice speaker didn't know. And Grace has a serious ear for language, like, the speed at which he picks up "Eridian" (I'll get to that) in the book is startling. But he's also human and learning an alien language based on nothing but gestures and their simple shared vocabulary. Rocky must be spending a lot of energy keeping himself from simply speaking Eridian as he usually does so that the computer can keep up and Grace too. His infallible memory definitely helps him remember what words Grace knows, but language is habit, it's a muscle and you don't think about the individual words when you speak. You just have a thought and it turns into words automatically. Eridians either don't have this automisation of repeated tasks in the way humans do, or Rocky is working overtime to keep his speech in check.

He speaks in English in Eridian. This is the big one. Most users on the English speaking internet are native English speakers and, knowing English speaking countries, most likely don't speak more than one language fluently (not an insult, just a statistic, that's just how it is when the entire world speaks your native language). It's therefore completely understandable that this part of Rocky's accommodation of Grace's slower learning went over most fans on this part of the internet's heads. And as someone from a part of the world where speaking two to three languages is standard, I still imagine it went over most of the people in my area's heads too because most people don't think about linguistics all the time like me. Anyway, to put it simply, translation is fucking hard. It seems simple, especially if you don't speak more than one, but it's not. Even highly related languages can end up nigh in comprehensible if you translate them directly into one another. The less related the languages are, the worse it gets. A lot of things cannot be translated directly because those types of words simply don't exist in the other language. Take Japanese. In a super simple basic beginner level Japanese sentence like "watashi no namae wa jon desu", you'd still struggle to translate it. And this is, like, almost comically expansive for how Japanese people actually speak in their daily lives. A causal, fluent Japanese sentence would be nonsense in English. Even this simple basic sentense would translate directly as something like "I my name about John is". The reason it's so weird is because Japanese uses particles to denote the roles of words in sentences, a lingustic concept that English doesn't have. Here "no" is possessive and indicates that the subject in front of it is the owner of what follows. English doesn't have anything even close to that type of structure and it can be really difficult for an English speaker to wrap their head around the sentence structure of Japanese when learning to speak it, not because they don't understand, but because constructing a sentence like that is so foreign. It takes a lot of mental effort to rewire your brain like that. Now imagine speaking English with Japanese sentence structure. Imagine doing that and never slipping into real English sentence structure because then your poor Japanese novice English speaker friend won't understand a word. It's not "my name is John", it's "I name John is" and that's giving you the benefit of not having to find replacements for the particles. That would be impossible. Now imagine you've met a literal alien species and have to start speaking English with alien language sentence structure. I cannot even begin to explain how wildly different Eridian and English likely are in every single way. Hell, they're so different that they're literally unpronouncable to the non-native speaker. And Rocky speaks Eridian with English sentence structure. It's explicitly pointed out in the book. And he does this flawlessly. Never slips up in any scene we get to see. I cannot begin to communicate the mental effort that would take for a human to do. Either all Eridians are crazy good at language or Rocky is just a turbo mega genius at it. Which is honestly not at all out of the realm of possibility.

All of this is to say that Rocky doesn't speak the way he does because that's just Eridian, that would be highly unlikely. He speaks the way he does because the language barrier is THICK and he's putting his entire rocky ass into making it thinner. It's less clear in the film, untouched as far as I remember, but in the book his speech becomes more nuanced as the story progresses. He uses more articles, more pronouns, more conjunctions, constructs more complex sentences, etc. As Grace learns more words and more about how Eridian as a language works, Rocky is fittingly upping the complexity of his speech.

It's never touched on but I have no doubt they spent many an evening (I know, I know, space, let me have this) talking about how Eridian expresses common English phrases and how to say this and that etc. etc. I'm probably the only person who'd find a long ass fic of just them discussing Eridian linguistic concepts and constructs interesting but hey, that's the nerd in me. I am the one writing this post, no one is suprised.

SO, anyway, to summarise, Rocky does a LOT of work to make his speech understandable to Grace. Grace starts to learn Eridian while they're still at Tau Ceti (not made clear in the film but is in the book) but he learns Rocky's frankensteined Eridienglish rather than real Eridian. I bet a lot of the trip to Erid was spent fixing Grace's understanding of Eridian so that he'd understand real Eridian.

By the time we pick back up with them in the biodome 16 years after arrival on Erid, Grace is fully fluent, meaning he must've already done all the hard work to learn proper Eridian which is unsurprising. He's a scientist, there's no way he'd accept just learning Eridienglish and never understanding any other Eridians than Rocky. Gotta be able to catch all the juicy gossip (what new science is being done and how to convince Rocky to convince the science Eridians to let him be part of it).