Learn Spanish (Latin America): Como no te pongas el pasamontañas, te van a reconocer. - If you do not put on the ski mask, they are going to recognize you.

When used with the subjunctive, como creates a conditional clause that expresses a warning or a threat. It translates to "if," but carries the heavier tone of "If you don't do this, [something bad will happen]." Because the consequence (te van a reconocer) is in the present or near future, we use the present subjunctive pongas. If the consequence were in the past, you would use the imperfect subjunctive.

Why is the word for ski mask masculine singular (el pasamontañas) even though it ends in -s?

Pasamontañas is a compound noun formed by combining a verb (pasa, from pasar) and a plural noun (montañas). Compound words made this way in Spanish are almost always masculine, and they use the exact same form for both singular and plural. So el pasamontañas is one ski mask, and los pasamontañas would be several.

Could I just say como no pongas el pasamontañas without the te?

No, because putting clothing or accessories on your own body requires the reflexive verb ponerse. If you drop the te and just use poner, it means "to place" or "to put" an object somewhere else (like putting a ski mask on a table), rather than putting it on yourself to wear.

Can I attach the pronoun te to the end of instead?