Aunque can be followed by El Imperfecto Subjuntivo. This happens in one of these two situations:
1. The information after aunque refers to a past action.
Using the subjunctive mood implies that the information has already been mentioned and is shared by the speaker and the listener. For example:
-La reunión de ayer con los clientes fue un éxito.
-Sí, pero aunque fuera un éxito, tuvimos que ceder demasiado por nuestra parte.
-The meeting with the clients yesterday was a success.
-Yes, but even though it was a success, we had to compromise too much on our side.
The second speaker uses El Imperfecto Subjuntivo because they both know that it was successful, so this bit of information is shared by both people involved.
2. The information after aunque refers to a future hypothetical idea. For example:
-No entiendo por qué el cliente no quiere firmar el contrato.
-Yo creo que aunque le ofrecieras más ventajas, no firmaría tampoco. Es un cliente muy testarudo.
-I don't understand why the client doesn't want to sign the contract.
-I think, even if you offered him /were to offer more advantages, he would still not sign. He is a very stubborn client.
The second speaker is using El Imperfecto Subjuntivo to express a hypothetical idea in the future.
However, aunque could also be followed by El Pretérito Indefinido in Spanish. In this case, the implication is that the information given after aunque is new information, mentioned for the first time by the speaker.
-El cliente firmó el contrato, aunque fue dificil convencerlo.
-The client signed the contract, although it was difficult to persuade him.