Modal verbs + which verb tense?

Question

When you use "could have", do you have to use past tense to be grammatically correct? These modal verbs really confuse me because I'm not sure if I always have to match everything to the tense of the modal verb I'm using, or if I can mix and match verb tenses. Please see my examples:

I could have used the paprika in the refrigerator since it is similar to the bell pepper the recipe called for.

I could have used the paprika in the refrigerator since it was similar to bell peppers the recipe called for.

I could have used what was in the refrigerator since they are similar to what the recipe calls for.

And then with "must have," I tried to use both simple past and present continuous. Do you think the sentence below is correct?

I must have ignored my friends who were cheering on for me because I was so happy to have reached the finish line.

What about should in this sentence?

I thought I should tell you that I will be attending your party next Friday.

Answer 1

Technically, the tense should match, so since we're using the perfect tense (conditional perfect in fact), you should use was.

But both could have ... is and could have ... was are acceptable in this case.

Why? Well, it's because your clause about the paprika being similar to the bell pepper might be true for a long time, and might continue to be true even in the present, so it might be okay to use is.

Maybe not. Maybe the paprika is rotten by now, or already eaten. Then is would make no sense. But a situation continuing to the present would justify use of the present tense is.

Consider:
"I could have gone shopping, because the supermarket was near."
"I could have gone shopping, because the supermarket is near."

If the supermarket has not suddenly moved, it probably still is near – it is now, and it was then – so either one is acceptable.

But some things do not last so long:
Correct: "I could have stayed longer, because it was early in the morning."
Incorrect: "I could have stayed longer, because it is early in the morning."

Unless you are describing something in the very recent past (minutes or hours ago), the fact that it is now early probably has nothing to do with the situation in the first half of that sentence, so mixing the past and present tense in this last example doesn't really work.

Answer 2

"I could have used the paprika in the refrigerator since it is similar to the bell pepper the recipe called for." This is correct in my view.

"I could have used what was in the refrigerator since they are similar to what the recipe calls for." This is also correct.

"I must have ignored my friends who were cheering on for me because I was so happy to have reached the finish line". This is correct as far as your question, however I'd say "...my friends who were cheering me on because...".

"I thought I should tell you that I will be attending your party next Friday." This is correct.

Answer 3

"I could have used what was in the refrigerator since it was similar to what the recipe calls for." is the sentence I would use.

The difference between the first and the second sentence is not grammatical. In the second sentence you are giving extra information: The paprika that was in the refrigerator is still in the refrigerator.

I would not compare paprika with bell peppers and say they are similar. Probably you mean that paprika has some property (e.g. color, taste) that is similar to the one the bell peppers suggested in the recipe have.

In the third sentence, you speak of something that was in the refrigerator, and then you use they. Apart in the case in a previous sentence you mention something else, it is not clear to what they is referring. It is wrong if they is referring to "what was in the refrigerator": In that case you should say "since it is similar to what the recipe calls for."