Have a look at the following sentence:

I will take a shower when I get home

We have two sentences: I will take a shower, and then when I get home. This last sentence is a secondary clause.

When, while, as soon as, after, before and until are conjuctions of time which introduce secondary clauses.

See how these sentences refer to future time but we use a present tense.

When my mom calls, I will tell her about my new job.

I will call you as soon as I get to New York.

While we are away, don´t forget to water the plants!

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