The use of present subjunctive

Question

It is certain that you will find a secretary who can use a computer.

The first part es cierto indicates that there is no use of subjunctive.

The part I have problem with is the part that says who can use a computer.

Should it be que pueda

Es cierto que encontrarás a una secretaria que pueda usar una computadora.

or puede?

Es cierto que encontrarás a una secretaria que puede usar una computadora.

2) the subjunctive is used to show doubt but also to show impersonal expression.

For this, I find the phrase, es verdad, kinda hard to categorize.

Should it be indicative or subjunctive mood?

Answer 1

Whether a clause needs subjunctive or not depends exclusively on (for subordinates) the clause immediately superior to it in the sentence hierarchy, (for relatives/adjective clauses) the noun that it modifies, and (for noun clauses) the surety and subjectivity thereof.¹

que encontrarás_{/encuentras/vas a encontrar} a un secretario is a noun clause which is described as cierto, therefore, it needs indicative, as we know it is certain and is not colored by emotion or value judgments.

que sepa usar un ordenador is a relative clause that modifies un secretario. We do not know who this secretary is as it's indefinite, therefore, it requires subjunctive.

You can generally safely ignore any part of a sentence other than the parent clause or head noun when determining whether you need subjunctive or indicative as mood doesn't cascade down the clause hierarchy. (this isn't to say that the rest of a sentence might not provide context you need to decide, just that grammatically speaking, they don't affect mood).

Answer 2

Nope, still indicative because you are certain that this person exists.

If your sentance had been:

I doubt you will find a secretary who can use a computer

You would have had to use the subjunctive...

Dudo que encuentres un secretario que sepa usar un ordenador

Because you doubt you will find this person, so they don't necessarily exist.