Learn Spanish (Latin America): Busco cualquier pluma que haga un trazo grueso. - I am looking for any fountain pen that makes a thick stroke.

The word cualquiera drops its final -a whenever it comes directly before a singular noun. It does this regardless of whether that noun is masculine or feminine. Because pluma is singular, we must use the shortened form cualquier.

Why is the subjunctive haga used here instead of the indicative hace?

We use the subjunctive haga because the speaker is looking for a pen that fits a description, but they don't have a specific one in mind yet. When the thing you are describing in a 'que' clause is unknown, indefinite, or might not even exist (which is heavily signaled here by cualquier), Spanish uses the subjunctive.

Why isn't there a word for 'for' after Busco?

The verb buscar already means 'to look for' or 'to search for' all on its own. Adding a preposition like por or para after it is a very common mistake for English speakers, but it is unnecessary and incorrect in Spanish.

What exactly does un trazo grueso mean?

The word trazo refers to a stroke or a line drawn by a pen, pencil, or brush. The adjective grueso means thick or heavy. Together, it just means the speaker wants a pen that writes with a thick line.