Question 1

Why does this sentence use como to mean "if" instead of si?

Accepted Answer

In Spanish, como followed by the present subjunctive (like tengas) is a special structure used to express a condition that carries a warning, threat, or negative consequence. You could just use si (Si no tienes el comprobante...), but como no tengas... adds a much stronger tone: "You'd better have it, or else you can't justify the expense."

Question 2

I learned that como means "since" or "because" at the start of a sentence. How can I tell when it means "if"?

Accepted Answer

The secret is the verb mood! When como means "since" or "because," it is followed by the indicative mood (the normal tenses you learn first). For example, Como no tienes el comprobante... means "Since you don't have the receipt...". But when como is followed by the subjunctive, it changes to a conditional warning meaning "If you don't..."

Question 3

Why is the second verb puedes in the normal present tense instead of the subjunctive?

Accepted Answer

The subjunctive is only triggered inside the specific warning clause introduced by como. The second half of the sentence (no puedes justificar el gasto) is the main result. Because the main clause states the factual consequence of the warning, it uses the standard indicative mood.

Question 4

Could I use recibo instead of comprobante here?

Accepted Answer

Yes, recibo and comprobante are often used interchangeably for a receipt. However, comprobante literally means a "proof" or "voucher." It is the most natural choice in formal or business contexts where you need to officially prove or justify an expense (justificar un gasto).

Como no tengas el comprobante, no puedes justificar el gasto.

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