As we discussed in imperative moods post in the Russian language there are three moods:
- Indicative mood - the straight-up sentences that describe events that did or will happen
- Imperative mood - for sentences that contain an order, and finally,
- Subjunctive mood - are for events one wishes to happen, or, could happen in some circumstances.
One can form subjunctive sentences by adding the particle "бы" to the sentence. The particle has no particular place in the sentence it has to be in, the only rule is that it is separate from the main verb it affects. Additionally, verbs in subjunctive mood are always in the past tense.
не мог бы ты закрыть окно, пожалуйста? Could you close the window, please я бы закрыл, если бы мне не лень было встать I would but I am too lazy to stand up