Is 'would be waiting' possible in this sentence?

Question

When I arrive at the airport tomorrow, my whole family _____ for me.

A. are waiting
B. was waiting
C. will be waiting
D. would be waiting

The answer is 'c'. Ok I understand that. But I wonder why D is wrong? In our country (non native) somtimes it is difficult to understand the concepts of 'will, would' or 'can, could'.

Would or Could is the past form of the modal verb 'will, can' but sometimes these words are used in past tense, present tense, even future tense. Very confusing for non navtive.

Of course I choiced 'c' for the answer but I wonder why not 'd'? For example I'm sure that my family will come tomorrow, in this case the answer is c, but my family is busy nowadays so, I'm not sure that my family will come or my family will not come. In this case could I use 'd'? I'm confusing.

Why can't 'd' be the answer? what is 'd' wrong?

Answer 1

Your sentence implies that your arrival at the airport is certain. However, "would be waiting" is conditional so it would require the first clause to be conditional too, for example an "if" statement:

If I were to arrive at the airport tomorrow, my whole family would be waiting for me.

If your arrival is certain, but it is your family's presence that is uncertain you would use "may" or "might" instead (these words are used pretty much interchangeably in modern English):

When I arrive at the airport tomorrow, my whole family might be waiting for me.

Answer 2

To form a sentence in the future with the conjunction "when, you use the main clause in the present simple or the present perfect and the subordinate/when clause with an auxiliary will/may/might. As there's no mention of may and might in the given choices, you should use "will".

As for the use of "would", it's the past of "will". You use it to refer to the future from the point of view of the past. For example:

He said he would meet his brother tomorrow.