Learning to spot and fix mistakes in complex French sentences can be tough. This is mainly because of tricky parts like subordinate clauses, relative clauses, and conjunctions. Here are some common problems learners run into:

1. Subordinate Clauses: These are parts of sentences that add extra information to the main idea. However, they can be easily misunderstood. Problems happen when learners miss the main idea or when the verb tense in the subordinate clause doesn’t match the main clause.

2. Relative Clauses: Using relative pronouns like "qui," "que," and "dont" incorrectly can make sentences unclear. It can be hard to pick the right pronoun based on the noun it refers to, leading to confusion.

3. Conjunctions: French has many conjunctions, such as "bien que," "pour que," and "parce que." This makes it harder to build sentences. Errors often happen when learners don’t connect clauses properly, which makes the sentence hard to understand.

Even with these challenges, there are ways to improve:

• Practice and Exposure: Reading a lot and looking at complex sentences can help you understand better.
• Analysis: Breaking down sentences to find the main idea and subordinate clauses can clear things up.
• Feedback: Getting suggestions from native speakers or people who are good at French can help you spot mistakes and misunderstandings.

While learning to handle complex sentences in French can be hard, regular practice and support can help you get better over time.