After fear and prevention, 'ne' does not always mean 'not'
In formal French you will often read sentences like je crains qu'il ne soit trop tard. The little word ne looks like the start of a negation, so English speakers tend to translate it as I fear that it is not too late. That is wrong. The sentence simply means I fear that it is too late. This ne is called the empty ne: it adds a formal tone but no negative meaning.
It appears after verbs of fearing, such as craindre and avoir peur, and after verbs of preventing or avoiding, such as empêcher and éviter. It is optional, it is typical of careful writing and formal speech, and it is almost always missing in casual conversation. You rarely need to produce it yourself, but you must recognize it when you read.
The pattern is always the same: a verb of fear or prevention, then que, then a clause in the subjunctive. In formal French, a bare ne can slip in before the verb of that clause. There is no pas, no jamais, no rien. The ne stands alone.
One way to understand it is that the speaker hopes the thing will not happen, and that negative wish leaks into the grammar. Logically, though, the clause stays positive. Remove the ne and the meaning does not change at all.
The real test is the second negative word. A genuine negation needs ne... pas (or another partner). Compare these two sentences:
Rule of thumb: a lonely ne after a fear or prevention verb means nothing negative. Ne plus pas means not.
Avoir peur is the everyday verb and craindre is the more formal one. The literary verb redouter works the same way. With craindre you will meet the empty ne very often, because both belong to the same formal style.
Éviter que and empêcher que both take the subjunctive and allow the empty ne. Here the result you want to stop is stated positively, exactly as in English.
Note that empêcher and éviter are more often followed by a noun or de + infinitive (éviter de tomber). The empty ne only appears in the que + subjunctive construction.
When the fear itself is denied or questioned, the empty ne normally disappears, because there is no longer a real fear pushing it in.
Reading je crains qu'il ne parte as I fear he won't leave. If you mean that, you need je crains qu'il ne parte pas. When writing, the safest choice at your level is to leave the empty ne out. Nobody will find it wrong.
You will also meet this empty ne after avant que and à moins que, and in comparisons of inequality. Recognize it there too:
After verbs of fearing or preventing (avoir peur que, craindre que, éviter que, empêcher que), a bare ne without pas is just formal decoration: translate the clause as positive. Only ne... pas really means not. Recognize the empty ne when you read, and feel free to leave it out when you write.