Expressing the future in time clauses and if-clauses

(a) Before I go to class tomorrow, I’m going to eat breakfast.
(b) I’m going to eat breakfast. Before I go to class tomorrow
In (a) and (b): before I go to class tomorrow is a future time clause.
(Before, After, When,  As soon as, Until, While ) + subject and verb = a time clause
(c) Before I go home tonight, I’m going to stop at the market
(d) I’m going to eat dinner at 6:00 tonight. After I eat dinner, I’m going to study in my room.
(e) I’ll give Rita your message when I see her
(f) It’s raining right now. As soon as the rain stops, I’m going to walk downtown.
(g) I’ll stay home until the rain stops
(h) While you’re at school tomorrow, I’ll be at work
The simple present is used in a future time clause. Will and be going to are NOT used in future time clause.
INCORRECT: Before I will go to class, I’m going to eat breakfast
INCORRECT: Before I am going to go to class tomorrow, I’m going to eat breakfast
All of the example sentences, (c) through (b), contains future time clause.
(i) Maybe it will rain tomorrow. If it rains tomorrow, I’m going to stay home.
In (i): If rain tomorrow is an if-clause
If + subject and verb = an if-clause
When the meaning is future, the simple present (not will or be going to) is used in an if-clause.