Yeah, people often confuse the past and future endings with the plural pronouns ni, chi and nhw as it's only one vowel difference:
Arhoson ni "We stayed" | Arhoswn ni "We'll stay"
Arhosoch chi "You stayed" | Arhoswch chi "You''ll stay"
Arhoson nhw "They stayed" | Arhosan nhw "They'll stay"
If it helps, the past tense ones all have o as the vowel, and -on rhymes with "gone" as in "been and gone".
The future tense vowels in order are "w-w-a" i.e. "ooh-ooh-ah", which sounds like a monkey. It helps some people remember!
Yn is used with definite nouns:
Arhosan nhw yn yr ysgol "They'll wait in/at (the) school"
Mewn is used with indefinite nouns:
Arhosan nhw mewn ysgol "They'll wait in/at a school"
If you want to be clear that they're "inside", then you can use (y) tu mewn i. This can be used with definite or indefinite nouns.
Arhosan nhw tu mewn i'r ysgol "They'll wait inside the school"
Arhosan nhw tu mewn i ysgol "They'll wait inside a school"