Think of apka as " your" apka khiyal or apka ghar, something that is about the other person or belong to the other person. here khiyal and ghar are sort of nouns.
apko is when doing something to the other person like "apko tang kerna" or "apko jagana". apko is being followed by i think what you call verbs.
Someone with a good grasp on grammer can explain better i think.
Same is for tumehien is to you in a sense somewhat informal form of apko.
and tumse is from you mostly. however it's different in mein tumse piar kerta/kerti hon,
Sentences in one language don't really translate in similar manner into another.
I hope i was helpful in someway and didn't end up confusing anyone.
Moreover, i rarely check this section, so if anyone posts a question here please drop me a message too, i will be glad to answer. This is the least I could do.