Plans and the future

The ending is three pieces. -(으)ㄹ turns the verb into a description of something still to come, 거 is the spoken form of 것 'thing', and 예요 is the copula from day 2. So 갈 거예요 is, underneath, 'it is a thing that will be done', a plan announced as a fact. The spelling is always 거예요, because 거 ends in a vowel and day 2's rule gives 예요, even though many Koreans write 거에요 by mistake.

에 pins an event to a point on the calendar or the clock: 세 시에, 토요일에, 주말에. 오늘, 내일, 어제 and 지금 point to a time by themselves and work like adverbs, so they never take 에. 내년 'next year' looks as if it belongs with them, but it is an ordinary noun and takes 에, so the safe approach is to learn the four that do not as a short list. English makes the same split without noticing: 'on Saturday', but just 'tomorrow'.

Because -(으)ㄹ 거예요 is a single ending on the verb, nothing else in the sentence changes: particles, 안 and 못 all behave exactly as before. The plain -요 present can also talk about the future, 내일 가요 'I am going tomorrow', much as English uses a present form for arrangements. The difference is emphasis: -(으)ㄹ 거예요 presents the plan as your intention, while the present sounds like a settled fact on the calendar. Both are correct, and you will hear both every day.