Since this is theoretical, wouldn’t the verb be ‘tengas?’ Confused!
I can’t explain why, but verbs such as ‘suponer’, ‘creer’, ‘pensar’, etc. are used with the indicative unless negated. This is something that differentiates Spanish from Italian, which favors the subjunctive.
I’m sorry, but I’m a little confused now (Spanish isn’t one of my stronger languages).
Doesn’t the explanation from @morbrorper relate to puedes (indicative) vs puedas (subjunctive).
Shouldn’t tener always be in the infinitive, with only the modal verb poder inflecting?
To add to what @morbrorper has already explained, I found this explanation in an old WordReference forum thread:
Sí es correcto el verbo suponer con indicativo.
Además del tiempo presente, se emplea el futuro perfecto o el futuro simple del indicativo para señalar incertidumbre o probabilidad en el pasado o en el presente.
Supongo que habrá salido . (Probabilidad en el pasado)
Supongo que estará saliendo en este momento. (Probabilidad en el presente)
Yes, but I read the original question as “why isn’t it ‘supongo que tengas razón’”?