Learn Italian: Lo sostituiremo per qualsiasi difetto. - We will replace it for any defect.

No, qualsiasi (meaning "any" or "whatever") is invariable. This means it never changes its ending, regardless of whether the noun is masculine or feminine. Note that it is almost always used with singular nouns, which is why it pairs perfectly with the singular noun difetto.

Could I use qualunque instead of qualsiasi in this sentence?

Yes, absolutely! Qualunque and qualsiasi are completely interchangeable in this context. Saying per qualunque difetto means exactly the same thing: "for any defect."

Why is Lo placed at the very beginning of the sentence?

Lo is the direct object pronoun meaning "it." Because sostituiremo is a conjugated verb (meaning it has a subject and a tense), the pronoun must go immediately before it. You only attach pronouns to the end of verbs if they are infinitives, commands, or gerunds (-ing words).

How is the verb sostituiremo formed here?

The base verb is sostituire (to replace or substitute). To form the future tense for "we" (noi), you simply take the infinitive, drop the final -e, and add the ending -emo. This gives us sostituiremo (we will replace). It is perfectly regular in the future tense.