Do you guys actually know when to use present/past perfect?

99% of native speakers don't know what present/past perfect is. We just intuitively know when to use it.

When there are two actions in the past and you want to communicate which happened first / the order. For example, “I was tired because I had not slept well that night”.

Exactly when to use which tenses can't be explained clearly, if at all. For most sentences, however, only one of them works. If you get it wrong it sounds ridiculous. I think this is something quite unfair about English, but it is the reality unfortunately.

I would say native speakers are not always consistent with how they use these tenses, but generally they have a pretty good intuition for when to use each one. Most people wouldn't be able to explain it well though, it's just a feeling.

op, i genuinely could not tell you what that actually means, and i’ve spoken this language for almost 20 years

I’m writing a book in past tense and regularly find myself writing in present tense and have to edit. Generally speaking, day to day it just comes naturally.

I still have to google "had had" and "that that" occasionally to make sure i'm not making grammatical errors in work emails.

Are you asking about difference between present/past perfect (have done vs. had done) or difference between perfect tense and simple past tense (have done vs. did)? For the difference between present/past perfect, the difference is very important for the meaning of the sentence. "Have you eaten lunch today?" is a normal question. "Had you eaten lunch today?" doesn't make sense. For the difference between perfect tense and simple past tense, the difference is more nuanced. "Have you eaten lunch today?" sounds like there is still time to eat lunch. "Did you eat lunch today?" sounds like lunch time has passed. There are also some regional differences. The UK uses present perfect more often than the US does. For example, someone in the UK may say "I've just eaten lunch" or "I just ate lunch", and in the US it's almost always "I just ate lunch". edit: I should eat lunch...

Native speakers do indeed know how to use the present and past perfect as well as all of the others. This is an important part of English and being able to convey your thoughts naturally and fluidly. As native speakers, we have been using this language since we were small children and have spent our whole lives completely immersed in a culture that uses it. I know the tenses and aspects can be challenging for learners because their languages likely make different distinctions or maybe none, but it is an important part of speaking English fluently. If you can’t use the aspects and tenses, you would not sound natural or fluent and you may be hard to understand, so it is truly important. A quick run-down is that the past perfect is used to sequence actions in the past. The past perfect happens first, before another action or before a point in the past. The present perfect is more difficult to understand, I feel, but in essence, it is used to talk about past actions that have present relevance. If you use the present perfect, you can’t use specific time points (e.g., yesterday, last week, etc.) unless it is ongoing (e.g., today, this week). It talks about actions that happened in the past, but the action relates to the present. Let me illustrate with some examples: Have you ever been to Brazil? -> There is an implied time period here: in your lifetime. Your life isn’t over so there’s a chance you will go to Brazil in the future, but they want to know if you have *already* been, so it’s asking about the past. Did you go to Brazil? -> This is focusing on one specific trip to Brazil that is over and done with. It’s not asking about your life experience. Here’s some context: Say you had a trip planned but your son got sick with COVID so you tell your friend, “I don’t know if we’ll be able to go”. Your friend is gone themselves for a little while so you don’t get in touch again until after you were supposed to be back from Brazil. They may ask “Did you go to Brazil?” or probably more likely “Did you end up going to Brazil?” They don’t want to know if you have *ever* been to Brazil and there’s no concern about whether you may in the future. It’s just about one trip that should be over. Have you found your keys? -> Say you’re looking for your keys because you had planned to go out with your friend and you were going to drive but they’re meeting you at your house. You tell them, “I’m not sure I’ll be able to drive because I can’t find my keys.” When they get there, they ask, “Have you found your keys?” Why? Because the keys are important to determining who’s going to drive or maybe whether you’re going to go at all. It has relevance to the present moment. Did you find your keys? -> Maybe you lose your keys, but this time, there’s no day out planned. You tell your roommate so they can be on the look out too. After a while you tell them you’re leaving. They say, “Did you find your keys?” Why? Because to them, there is no present relevance. You’re leaving whether you found them or not and they’re not leaving either way. Whether you found them has less relevance here.

If you asked 80% of native English speakers what the difference is they wouldn't be able to tell you. The other 20% are writers, teachers, and people who paid attention in high school or have learned other languages. Everyone knows how to use them, though, which goes to show how many people know very little about the language they can communicate perfectly in.

It is just sort of intuitive must of the time for native speakers. There's are even some instances where either one can be used with little to no change in meaning

I know what each one is and how to use it but I couldn’t explain the grammar behind it

I'd look at past perfect like this: Imagine some event happened sometime in the past. Past perfect talks about some action being completed before that past event. "I went to the supermarket to buy bread but when I got the cash register, I realized I had lost my wallet." You can use simple past in a lot of cases where context is clear. This is probably most cases. If you absolutely need to make the order of several past events clear, past perfect helps.

A lot of English speakers may not know grammatically what the tenses are, but they would naturally know how and when to use them even if they couldn't explain why.