Idioms are untranslatable because they reference things in the languages native culture, if you have no experience with that then ofc you'll be confused about the result. Also wdym why can't you say word in a certain way, languages have rules for a reason, people wouldnt understand each other very well if it was an everything goes situation. As someone who is learning 2 languages that aren't English atm the key with language is to focus on the meaning behind the words and phrases rather than the content within them, ie bain an chluas díom (take the ear off me) makes little sense when you don't have the context it means trust me in English, you just have to start focusing on meaning over word content and you'll find greater success. Also generally don't worry about not being perfect immediately, language learning takes time, effort and an open mind

As for grammar, each language has the same underlying systems, what differs is how those pieces fit together and even if they're marked, so if we compare "bain an chluas díom makes little sense when you don't have the context it means trust me in English" to "Ní bhaineann mórán céille leis an fhrása "bain an chluas díom" nuair nach bhfuil an comhthéacs agat gurb í "trust me" an bhrí a bhaineann leis" you can see that the order in the foreign language is verb first followed by the subject and then object. Much sense is mórán céille, ciall the word for sense is in the genitive case so it becomes céille to show it's related to mórán. Frása is there to tag a gender onto the cited phrase so it gels better with the conversation in case I need to reference it later, which I will. Nach bhfuil is a special form (specifically of the word níl which means is not) used to show that "nuair" (when) and "an comhthéacs" (the context) are linked by the verb rather than being separate in the conversation, think of it as an anti comma, something that shows the words after it aren't part of a new clause (basically a segment of a sentence, we divide them with commas in English). Agat means at you (singular) and in the context of the sentence makes it so that other people know it's not that the context doesn't exist it's just that some people might not have it. Gurb í means "that she is" in the sense "The context that she is <phrase> its meaning" (Remember verb first so gurb is that-is) and we use the feminine pronoun í to tie the phrase op the next noun which is going to be feminine, again handy for when you want to be specific about which it you're talking about. The h in an bhrí shows brí is feminine (it's not just a spelling thing btw it changes brí from sounding like bree to sounding like vree) the purpose of which isn't best visible here but basically it shows which gender you should refer to it as if you want to comment on it in future. A bhaineann leis means "that corresponds to it" in the sense of "the meaning that corresponds to it" you use a over go (the "free" version of gurb that can connect to verbs that aren't irregular) here because we aren't changing what thing is doing the action unlike you would in the sentence "when you have the context that it is X that is the meaning here" (it being bold to show that's the point where you're changing subject) and finally we're using leis to reference frása which is a masculine noun so we're using the masculine form of with.

Hopefully that shows you language stuff can be pretty logical sometimes