1, can you put it at the beginning of the sentence to guide the subject clause, but if not.

2.When can be used after prepositions to guide object clauses, but if can't.

When introducing appositive clauses after nouns, use when instead of if.

After some verbs, you can only use when instead of if to guide the object clause.

Extended data

If usage

As a subordinate conjunction, if can guide conditional adverbial clauses, adverbial clauses and noun clauses. Details are as follows:

1, if conditions can be expressed. If the main clause uses the future tense, the if clause uses the present tense to indicate the future.

2, if can represent virtual, if, if.

3.if-guided conditional clauses can omit be and subject.

4.if can guide noun clauses and make a "whether" solution, which is often placed after verbs such as ask, doubt, know, learn, see and think. Generally used in spoken language, not at the beginning of a sentence.

The usage of whether or not

1, where is used as a conjunction, indicating "whether, isn't it", which can guide noun clauses or infinitive phrases.

2.when can also lead adverbial clauses, meaning "whatever, it doesn't matter", and the future tense is usually replaced by the ordinary modern tense in the clause.