Learn Italian: Siamo più deboli di voi.

In Italian, you don't add an ending like "-er" to make comparisons. Instead, you use più (more) before the adjective. So, "weaker" is literally expressed as "more weak": più deboli.

Why does di mean "than" in this sentence?

When comparing two people or groups using più (more) or meno (less), Italian usually uses the word di to mean "than" right before a noun or a pronoun like voi (you). So, più... di voi means "more... than you".

Why is the word for weak spelled deboli instead of debole?

Adjectives have to match the number of people they describe. Because siamo means "we are" (a plural group), the adjective must be plural too. Adjectives that end in -e in the singular, like debole, always change their ending to -i in the plural to become deboli.

Where is the word for "we" in this sentence?

In Italian, the subject pronoun noi (we) is usually left out because the verb siamo already tells us exactly who is doing the action ("we are"). You could say Noi siamo più deboli di voi, but dropping noi is much more common and natural.