Learn Italian: Le perle che stai usando con la lana sono molto strane. - The pearls that you are using with the wool are very strange.

Stai usando is the present continuous tense, made by combining the verb stare (to stay/be) with the -ando ending for an -are verb like usare. Just like in English ("are using"), it emphasizes that the action is happening right at this exact moment. While usi is also correct, stai usando sounds more immediate.

Why does this sentence use che for "that" instead of quello?

Che is a connecting word (a relative pronoun) used to link two halves of a sentence together, acting like "that", "who", or "which" in English (the pearls that you are using). You only use words like quello when you are specifically pointing at something, like saying "I want that pearl" (Voglio quella perla).

Why does the sentence end with strane rather than strano?

Adjectives in Italian must match the noun they describe in both gender and number. Here, the adjective is describing le perle (the pearls), which is feminine and plural. The dictionary form of the adjective is strano (strange), so it changes its ending to -e to become the feminine plural strane.

Since this lesson is about polite requests with potresti (could you), how would I use it to ask the person to use different pearls?