This is one of those situations where English and Welsh do things slightly differently; in this case it’s to do with what tense is used after “if”. Take for example a sentence like “if you go out today, please buy some bread”. The “if you go out” bit is actually referring to the future - the hypothetical going-out hasn’t happened yet, but if it does, it’ll happen in the future. So in this situation, unlike English, Welsh actually uses the future tense, saying in effect “if you WILL go out” rather than just “if you go out” (os fyddi di’n mynd allan). It sounds a bit odd in English because we don’t ever use the future tense with “if”, but it’s quite common in some other languages, including Welsh.
By contrast, if you take an example such as “if you’re happy and you know it, clap your hands”, the hypothetical happiness is right now - the speaker doesn’t know if you’re happy, but they’re wondering if you are right now. So in that case, Welsh would use the present - os wyt ti’n hapus.
But don’t worry about it too much, because even native speakers don’t do this consistently every time and it’s not something that would ever prevent you from being understood.