Questions & Answers about Ich weiß, dass er übernachten würde.
Why is it würde here instead of wird?
würde is the Konjunktiv II form, which means would in English. It shows a hypothetical situation. If we used wird, which is the regular present/future tense, the sentence would mean I know that he will stay overnight.
Why are both verbs (übernachten and würde) stacked at the very end of the sentence?
The word dass (that) starts a subordinate clause, which has a strict rule: the conjugated verb must go to the very end. Here, the conjugated verb is würde. The infinitive verb übernachten also goes to the end, but the conjugated verb claims the absolute final spot, giving us übernachten würde.
Could I use bleiben instead of übernachten to say he would stay?
You could use bleiben, but it just means to stay in general. übernachten specifically means to stay overnight or to spend the night (think: über + Nacht). It is a very common, single-word verb for this exact situation.
Why is dass spelled with a double 's' here?
dass is a conjunction meaning that (as in I know that...). The word das with a single 's' is the neuter article the or a pronoun meaning this or that one. Since we are connecting two clauses, we must use the conjunction dass.