Learn Italian: Il cavaliere stava pulendo la spada di metallo. - The knight was cleaning the metal sword.

stava pulendo is the past continuous (the imperfect of stare + the gerund). It sets the scene by describing an ongoing, background action ('was cleaning'). The passato remoto is used for sudden, completed actions in the distant past. If something interrupted the knight—like a dragon suddenly arriving—that new event would use the passato remoto (for example, un drago arrivò).

pulire is an -isc- verb (like io pulisco). Does that affect how we make the -endo form?

Not at all! The gerund (the '-ing' form) is very regular in Italian. For all regular -ere and -ire verbs, you simply drop the ending and add -endo. The -isc- pattern only affects the present tense and the imperative, so here it is simply pulendo.

Why is it spada di metallo rather than translating 'metal sword' in the same order as English?

In English, we often use a noun as an adjective by placing it right before another noun ('metal sword', 'water bottle'). Italian does not allow this structure. Instead, you use the main noun (la spada) followed by di (of) and the material (metallo).

Is there a connection between cavaliere (knight) and the word for horse?