Learn French: Le projet est plus astucieux que je ne l'envisageais. - The project is more clever than I envisioned.

This is the ne explétif. In formal and written French, it is often added after affirmative comparisons of inequality (like plus... que or moins... que). It doesn't make the verb negative; rather, it highlights the gap between reality and your expectations. It isn't translated into English, and you will often hear native speakers drop it in everyday conversation.

What exactly is the l' replacing in je ne l'envisageais?

It acts as the neutral object pronoun le (shortened to l' before a vowel). Instead of replacing a specific noun, it stands in for the adjective astucieux or the general state of being clever. A literal translation would be 'than I envisioned it to be'.

Why use the imperfect tense (envisageais) here instead of the passé composé (ai envisagé)?

The imperfect is used because 'envisioning' the project was an ongoing state of mind or an assumption you held over a period of time before seeing the final result. The passé composé would sound like a single, isolated action, which doesn't fit the context of a prolonged expectation.

What is the difference between astucieux and intelligent?