Rules of tense agreement

Question

Do the tenses match in the following context?

• I have done as much as I can/could do to get selected for the job, still I have failed.

Should I use can or could there?

Answer 1

Usually when could is used in the present tense, it is either used to ask for a request or to state a possibility. For example:

1. That could be Tom’s brother. He looks like Tom. (possibility)

2. Could I use your computer to email my boss? (request)

I think can would be a better choice for your sentence.

I have done as much as I can do to get selected for the job, still I have failed.

By the way, in order to improve your sentence, it's better to omit do after can. Because the meaning of the verb "do" can be easily understood from the previous part of your sentence.

I have done as much as I can to get selected for the job, still I have failed.

Answer 2

It depends on what you would like to emphasise.

• When discussing the general ability, I would use can, which is the indicative form.

I have done as much as I can do to get selected for the job, still I have failed.

means

I have done as much as I am able to do, still I have not been selected.

• When I want to emphasise the possibility, I would use could, which is the past subjunctive form. This form is used when discussing possibility or irrealis.

I have done as much as I could do to get selected for the job, still I have failed.

means

I have done as much as possible to do to get the job, still I have not been selected.

This page has a good explanation for distinguishing between can and could. I hope this helps!