Learn Polish: Myślę, że on jest silniejszy od ciebie. - I think that he is stronger than you.

In Polish, a very common way to make a comparison is to use the preposition od (which usually means 'from') followed by the Genitive case. So literally, this sentence translates to 'he is stronger from you'.

How do we get silniejszy from the basic word for 'strong'?

The base adjective is silny (strong). To make it comparative ('stronger') for a masculine noun like on, we drop the -y ending and add -ejszy, giving us silniejszy.

I learned that 'you' is ty or cię. Why is it ciebie in this sentence?

Because of the preposition od, which requires the Genitive case. The Genitive of ty has two forms: a short one (cię) and a long one (ciebie). You must always use the long form (ciebie) when it comes directly after a preposition.

Can I leave out że like we often leave out 'that' in English?

No, you can't. In English, saying 'I think he is...' is perfectly fine, but in Polish, the conjunction że (that) is mandatory to connect the two parts of the sentence. You must always say Myślę, że....