A : What time will you phone me tomorrow? B : I'll phone you when I get home form work. 'I'll phone you when I get home from work' is a sentence with two parts → I'll phone you → when I get home from work (tomorrow) The time in the sentence is future ('tomorrow') but we use a present tense (get) in the when-part of the sentence. The same thing happens after : while before after as soon as until ..