Learn German: Plötzlich wusste ich es, denn ich habe den Fehler bemerkt. - Suddenly I knew it, because I noticed the mistake.

wusste is the simple past (Präteritum) form of wissen (to know a fact). While spoken German usually prefers the two-part past tense (Perfekt) for most verbs, a handful of highly frequent verbs like wissen, sein, and haben are almost always used in the simple past instead, even in casual speech.

Why is the order wusste ich instead of ich wusste?

Because the sentence starts with the word Plötzlich (Suddenly). In German, the conjugated verb must always be in the second position in a main clause. Since Plötzlich takes the first spot, the verb wusste must come next, pushing the subject ich right after the verb.

Why doesn't denn push the verb to the end of the sentence?

Both denn and weil mean 'because', but they affect word order differently. denn is a connecting word that takes 'position zero'. This means the word after it (ich) counts as position one, keeping the helping verb (habe) in its normal second position. If you used weil, the verb would be kicked to the very end: ... weil ich den Fehler bemerkt habe.

Why is it den Fehler and not der Fehler?