People who learn to speak French fluently at an adult age often stop paying attention to the mechanics of learning. They feel comfortable in the language. They can make themselves understood. No one corrects them. Unfortunately, the big danger here is something linguists call fossilization.
There are two sides to fossilization. First of all, certain bad habits have become so ingrained that you are unaware of them. Here are some examples:
French has many words that are identical to English words but are pronounced differently. The French pronunciation of le budget and la suggestion is very different from the English version.
You know about masculine and feminine nouns in French. No problem. You think that véhicule is feminine because it ends in -e and the word molécule is feminine (la molécule). So, you say la nouvelle véhicule. This is wrong. Despite all appearances, véhicule is masculine and one should say le nouveau véhicule.
A related problem is how to get gender agreements right. This is usually not a big problem when the adjective comes before the word. Most people get la nouvelle politique right. The problems arise, however, when the adjective comes after the word, especially if it is separated by some other words. What happens is that once the main word has gone by, we simply forget about its gender. For example, one hears l’histoire américain instead of l’histoire américaine, des informations politiques intéressants instead of des informations poltiques intéressantes.
This problem is worse the when adjective is very far in the sentence as in: La politique économique du nouveau gouvernement est, à mon avis, très intéressant (should be interessante) ‘The economic policy of the new government is, in my opinion, very interesting.’ This sort of mistake is very hard to correct because it requires being always aware of key grammatical points.
Another common ingrained mistake is things like j’ai parti (je suis parti) or j’ai venu (je suis venu). Here, of course, the problem is the wrong auxiliary avoir instead of être. Again, this sort of mistake is hard to correct because many people are totally unaware of it.
The other side of fossilization is simply avoiding certain areas of grammar because they are thought to be too difficult. For example, many people never use the simple future tense because it is complicated. Instead of j’irai ‘I’ll go’, tu verras ‘you’ll see’, elle viendra ‘she’ll come’, they use je vais aller ‘I’m going to go’, tu vas voir ‘you’re going to see’ and elle va venir ‘she is going to come’. By using aller with the infinitive, you avoid having to figure out the future tense of the verb.
Another common area of avoidance is something called the subjunctive. This is a rather complex verb form used in expressions like il faut que vous sachiez la vérité ‘you have to know the truth’. Many people just avoid the subjunctive completely and end up either making major mistakes or just working around it.
What can be done if you feel that you may be fossilized? Remember that most people never realize that their French is somewhat lacking. But if this is a major concern of yours, the best course of action is to get some outside help. With the kind of mistakes discussed here, it is nearly impossible to correct yourself.
Working with a tutor is a good idea. Especially a tutor who is experienced with advanced students and with the problem of ingrained errors.
An excellent solution is a class in written French. The written language is much less forgiving than the spoken language. Writing French correctly requires great attention to these details of gender agreement and verb forms.