Learn Spanish (Spain): Él encontró un arma oxidada en el bosque.

Arma is a feminine noun, but because it starts with a stressed 'a' sound, it takes the masculine singular article un (or el) to make pronunciation smoother and avoid blending two 'a's together. This is why you see un arma here.

Wait, if it is un arma, why is the adjective oxidada feminine?

Even though we use the masculine article un for pronunciation reasons, the noun arma remains fundamentally feminine. Any adjectives that describe it must match its true gender, which is why we use oxidada (rusty), not oxidado.

If he had found multiple rusty weapons, how would this sentence change?

It would become Él encontró unas armas oxidadas en el bosque. The rule about using a masculine article only applies to the singular forms. In the plural, the 's' in unas breaks up the two 'a' sounds, so we go back to using the normal feminine plural articles.

The present tense is encuentra (he finds). Why doesn't encontró have a stem change here?

The verb encontrar has an o-to-ue stem change in the present tense, but it behaves completely regularly in the preterite (simple past) tense. For regular -ar verbs, stem changes do not carry over to the preterite, giving us the standard conjugation encontró.