Why can your teacher/fluent friends never tell you the Gaelic for things?
Do you ever notice that you will ask your Gaelic teacher or your Gaelic-speaking friends for a word and they will shrug or say they can't think of it? And yet, they have no trouble speaking the language themselves, and you know that they must be talking about similar things to the word you were looking for. Where does this odd-seeming disconnect come from?
On the other hand, do you ever learn a word from a vocabulary list or look something up in a dictionary and then try it out, only to find that people are making funny faces at you? Maybe they are even laughing or simply shaking their heads in bafflement, with no idea what you meant. Why did that vocabulary list or dictionary not work the way you intended?
More recently, you tried plugging something into Google Translate, but you found that the Gaelic it offered you was disdained by your Gaelic-speaking friend. Why didn't Google give you the correct word?
Well, in languages, as in so many other things, context is key.
Some years ago, I was teaching a class of fairly advanced students. One of them looked up a dictionary for a word to finish a writing task he was working on. He started writing down the word and then paused, laughing slightly. When I asked him what was up, he explained that he had looked up the word 'plumb' on the English side of the dictionary. He was looking for the word that is equivalent to 'level' (he was writing about decorating a house), but he had started writing down the word for the fruit before he noticed his mistake. Thankfully, he spotted the problem and corrected it himself. [Edit: see the comments and imagine my facepalm to your heart's content.]
That specific problem was due to there being two identical words in English, exact homonyms that are indistinguishable based on either sound or spelling. Thankfully, in English and Gaelic, exact homonyms are not too common (although, I do wish you luck with all the a and na words when you are working on your Gaelic!). When learning our vocabulary from aural sources, we would also have to be careful with homophones: i.e. words that sound like other words, although their spellings differ (think of main and mane in English). Again, though, these are relatively uncommon and shouldn't cause you too many problems.
To avoid this kind of mistake, we used to teach students what we called 'dictionary skills' or 'dictionary awareness'. This would involve encouraging them to familiarise themselves with the dictionary they were using and make sure they understood the full information that a dictionary entry was giving you (i.e. part of speech, usage examples, the order of meanings, connotation, etc.). We would also insist that students never rely on unidirectional dictionary use: in other words, if you looked up the English headword and found a possible Gaelic term you might use, you should then immediately go ahead and look up that Gaelic headword in the other half of the dictionary or in a different dictionary. Very often, that simple act of double-checking would save you from a clanger. Similarly, checking three or four dictionaries would potentially also be helpful. For instance, if you look up a word and Dictionary A is suggesting one translation, but Dictionaries B, C and D have no mention of that translation, you might develop a healthy scepticism for that choice.
A bigger issue than the homonyms and homophones is with what we call 'polysemy'. A word is polysemous if it has more than one meaning. What if we learn that a particular word means one thing and then we seem to hear that word being used in a way that that meaning couldn't possibly make sense? Or, what if we learn a very specific, context-bound meaning of a word and then we try to use that word with that same meaning in a completely different context? These things cause us a lot of problems when learning languages (just as they caused us communication issues when we were growing up and still working on mastering our native language).
With all this in mind, it is very important to make sure that you learn vocabulary in context wherever you can. The actual meaning that a word carries in context is usually at least slightly different from the meaning it appears to have when you find it in a dictionary. Some people begin to develop a sense that written meanings are not reliable at all and that they should get their vocabulary from a native speaker friend. Of course, this does work. However, the same problems still exist, unless your friend is both incredibly knowledgeable and also has unparalleled patience.
Therefore, by far the best way to acquire vocabulary, whenever you can possibly manage to do it, is in authentic context. What do I mean by authentic context? I mean, in conversation or in reading genuine texts. The more conversing and reading you do, the more you will encounter words and phrases being used correctly. Try to notice the whole phrase, rather than just the word. Try to notice the way the phrase fits into whole sentences. And try to notice the way the sentences fit into discourses and texts. The better you get at noticing the detail of how other people use Gaelic, the better you will get at making good selections yourself when you look for vocabulary.
This principle applies not only to vocabulary. Context is also key when it comes to both grammar and the sound system of the language. We always tend to imagine that grammar is a set of immutable rules that must be mastered and used in a specific, set form. There is only a modicum of truth in this. In fact, native speakers are generally relaxed about 'fiddling' with grammar in order to get their points across or to achieve a certain kind of expressive effect. Listen to them talking and pay attention to your reading: work out where the grammar rules are flexible. Similarly, we often start out in our learning process by trying to master individual phonemes and then we put these together to form words. However, once words appear with other words, the sounds immediately start to change. Again, you have to listen to the patterns and rhythms and try to tune your ear in so that you become more mindful of the ways the sounds change.
Once you develop these skills, you will discover that you have become a much more independent Gaelic-learner. Moreover, you will be better at asking the right questions when you need to pester your friend for vocabulary or when you need to look up the dictionary.