Learn Spanish (Spain): Ojalá seas más considerado con tu compañero de piso. - I hope you are more considerate with your roommate.

The word ojalá (meaning "let's hope" or "if only") triggers the subjunctive because it expresses a wish or desire, not an established fact. We use the present subjunctive seas here because the speaker is hoping for a change in behavior in the present or near future.

Since our current lesson is about past reproaches, how would I change this sentence to say "You should have been more considerate"?

To express this as a past reproach about an unfulfilled obligation, you would use the conditional form deberías (you should) followed by haber (have) and the past participle sido (been). The sentence becomes: Deberías haber sido más considerado con tu compañero de piso.

Is compañero de piso the only way to say "roommate" in Spain?

In Spain, compañero de piso literally translates to "flatmate" and is the standard term for people sharing an apartment. If you share an actual bedroom with someone, you would use compañero de habitación. In Latin America, you will often hear compañero de cuarto or simply the English word roomie.

If I am speaking to a woman, which words in this sentence need to change to feminine?

You would change the adjective to considerada because it describes the person you are speaking to. If her roommate is also a woman, you would change the noun to compañera, making the sentence: Ojalá seas más considerada con tu compañera de piso.