Here’s the rule:
Use the present tense to indicate customary behavior, no matter when it happens.
That’s right, when you do something, something happens. Doesn’t matter whether you’ve actually done it yet. Here are a few examples:
When true, the boolean indicates acceptance; when false, it indicates rejection.
When you press Enter, the window appears.
When I get hungry, I eat!
Tomorrow I go to the dentist.
Every day I get up before 6:00.
Maybe you’re writing instructions for operating a machine that hasn’t even been built yet. Use the present: When you turn the key, the engine starts.
When may you use the future? I can think of four times:
- When you want to be vague: Someday we will get married.
- When the time is important: Tomorrow I will go to the dentist, but the next day I won’t
- For a command: You will clean your room! (In military tech writing they use “shall.”)
- When something isn’t customary: We will go on the trip if we can ever get the car started.
By the way, this is a good rule even if your writing isn’t strictly technical.