Learn French: L'adversaire est plus prévisible que tu ne pensais. - The opponent is more predictable than you thought.

This is the ne explétif, which is exactly what this lesson covers! It appears in affirmative comparisons of inequality (using plus... que or moins... que). It doesn't negate the verb; instead, it highlights the gap between reality and expectation. You'll hear it in careful speech and see it in writing, but it's often skipped in casual conversation.

Why use the imperfect tense pensais instead of the passé composé (as pensé)?

The imperfect is used here because your "thinking" was an ongoing state of mind or an opinion you held leading up to this moment. The passé composé would sound like you only had a single, sudden thought about the opponent's predictability.

Is it okay to add a pronoun and say que tu ne le pensais to mean "than you thought it"?

Yes, absolutely! Using the neutral pronoun le to represent the idea that the opponent is predictable is perfectly correct. However, with verbs of opinion like penser or croire in these comparison structures, the le is very frequently dropped, as seen in this sentence.

How does the word prévisible translate to "predictable"?

It comes from the verb prévoir, which literally translates to "to foresee" (pré- for "before" and voir for "to see"). Adding the suffix -ible makes it "foreseeable," which is used exactly like "predictable" in English.