How can I replace certain strings of symbols automatically? Please, read the description of my problem, I already know macOS can change certain individual strings. What I want is different.

I bought a MacBook from Ebay.com with macOS High Sierra, and being Spanish, I normally use the Spanish keyboard distribution (because I need to type things like ñ, á, é, í..., which would be very slow to type using Option + e + letter). The problem with this is that most symbols don't match what I see on my physical keyboard, and using the Spanish distribution I actually have to memorize which keys do what. Therefore, I've tried to change the keyboard configuration so that something like 'a (easy to type, uses the same keys as á in Spanish) gets changed automatically to á. But it only works when 'a is typed like that, and not when it's part of a word (like lloverá).

Could I somehow use the English distribution (so that I can actually see in my keyboard what every key does) but have things like 'a or 'n automatically changed to á and ñ?

Thanks.
Go to system preferences/keyboard/input sources and add the keyboard called US International PC to the list.  Then remove the others.  This keyboard turns ` ~ ' " ^ into dead keys to add accents.  So 'e produces é, ~n produces ñ.