Incorrect: He asked had we taken our food.
Correct: He asked if we had taken our food. OR He asked whether we had taken our food.
Incorrect: I am not sure will I have time.
Correct: I am not sure if I will have time.
Incorrect: I asked whether had she any letters for me.
Correct: I asked whether she had any letters for me.

Explanation

Indirect yes/no questions are usually introduced by if or whether. Note that in an indirect question we put the subject before the verb.

Incorrect: He said that he saw her last year.
Correct: He said that he had seen her last year.
Incorrect: I could not meet him because he went out before I arrived.
Correct: I could not meet him because he had gone out before I arrived.
Incorrect: He got angry even before I said a word.
Correct: He got angry even before I had said a word.
Incorrect: There I met a man who was my classmate ten years ago.
Correct: There I met a man who had been my classmate ten years ago.
Incorrect: This was going on for a long time.
Correct: This had been going on for a long time.

Explanation

These are examples of the failure to use past perfect tense when the time of one past action is more past than that of another.

Incorrect: I will call you when dinner will be ready.
Correct: I will call you when dinner is ready.
Incorrect: I will write after I returned.
Correct: I will write after I return.
Incorrect: When I will go to New York I will meet him.
Correct: When I go to New York, I will meet him.

Explanation

When the principal clause is in the simple future tense, the subordinate clause should be in the simple present tense. Note that when a subordinate adverb clause begins the sentence, we separate it from the rest of the sentence by a comma.

Incorrect: If I would have done this I would have been wrong.
Correct: If I had done this I would have been wrong.

Explanation

In a Type 3 Conditional sentence we use had + past participle in the if clause and would have + past participle in the result clause.

Incorrect: She knows to knit.
Correct: She knows how to knit.