The subjunctive is a grammatical mood that expresses suggestions, wishes, etc. It also expresses doubts. When a sentence does not express doubt. The grammatical mood is not subjunctive.
E.g.
•Dudo que tengas dinero para mi almuerzo.
This sentence expresses doubt.
Whereas..
•No dudo que te mantienes en buena forma física.
The sentence does not express doubt.
Ustedes tienen unas bebidas.
De vez en cuando pagaba las cuentas del hotel con tarjeta de crédito.
If the starting tense is future and in the ellos form and its a regular verb the answer would be comprarían.
