Litional.
1. if you practice your english often, (to improve)
2. if i looked like him, (to be)
3. if we had taken the earlier train, (to arrive)
4. if it rains on their wedding day, (to cancel)
5. if you read this english book, (to learn)
6. if she had practiced speaking.ito pass

These are "if clauses":

1. If you practice your English often, you will improve it.

2. If I looked like him, I would be him.

3. If we had taken the earlier train, we would have arrived.

4. If it rains on their wedding day, they will cancel it.

5. If you read this English book, you will learn English.

6. If she had practiced speaking, she would have passed.

Explanation:

1. In the 1st sentence, there is a recommendation from the speaker and present tense is used in the "if clause" to refer to a future, conditional event. The person's English will improve on condition that they practice their English often.

In future "if clauses", present tense should be used after "if".

2. Here, the speaker is referring to a future potential event, to look like someone else. Meaning = if, in a parallel universe, this person would have looked like someone else, they would be that person.

In this conditional "if clauses", past tense is used after "if" and "would" + verb is used in the 2nd sentence.

3. In this sentence, the "if clause" is referring to a past event, something that they could have changed, but they can't anymore. So past perfect is used after "if" and  "would" + "have" + past participle is used in the 2nd sentence.

4 + 5. These "if clauses" express someone's opinion as a given fact. So present is used after the "if clause" and future in the 2nd sentence, because the event is in the future.

6. This "if clause" refers to a past event, the possibility for that person to have passed the test. Since it's a past event, past perfect is used after "if" and then "would" + "have" + past participle of the verb is used in the 2nd sentence.