What is a time clause?
Time clauses are preceded by adverbs or adverb phrases which show that they represent a time.
Examples of these adverbs or adverb phrases are: when, before, after, as soon as, until.

A time clause shows that an event will happen at a certain time. Although they have a subject, verb and object, these clauses simply point to a time, similar to saying "at 7 o'clock".

We use the present tense to talk about future times.
The future tense is used in the main clause.
The present simple tense is used in the time clause.
When two clauses are joined by adverbs of time or adverb phrases, the future form should not be repeated.

• Tom will turn off the lights when he leaves the office. (Not when he will leave).
• Dad will reply to the letter when he comes home from work. (NOTwhen he will come home.)
• I will tidy the living-room before I go to bed. (Not before I will go).
• They will do the shopping before we arrive. (NOT before we will arrive.)
• She will go running after it stops raining. (Not after it will stop raining.)
• Alex will write a review after he finishes reading the book. (NOT after he will finish reading the book.).
• I will not go home until I finish the report. (NOT until I will finish).
• The nurse will wait until the ambulance arrives. (NOT until the ambulance will arrive.)
• Jane will call her mother as soon as she arrives at the airport. (Not as soon as she will arrive).
• The operation will begin as soon as the patient is ready. (NOTas soon as the patient will be ready.)

Time clauses with reference to the future can also be introduced by other expressions such as:
once, immediately, the moment, the minute, the day, by the time ...

• Tony will call the restaurant the moment he gets home.
• I'll contact you once I receive an estimate.
• You will be notified the minute your order arrives.
• It will be dark by the time we arrive home.

The time clause may come before or after the main clause with no change in meaning. However, if the time clause begins a sentence, a comma is placed after it.

• The moment I hear the doorbell, I will put on my coat.
• I will put on my coat the moment I hear the doorbell.
• The day you graduate, your mother will bevery happy.
• Your mother will be very happy the day you graduate.