How are fear clauses constructed?

The general formula for a fear clause is:

"Verb of fearing" + 'ne' + verb in the subjunctive mood

e.g : timeo ne veniat "I fear that he is coming"

Notice how 'veniat' is in the present subjunctive as it is following 'timeo' which is in the present indicative. It is following the sequence of tenses.

If we were to put this phrase in the past it would become:

timui ne veniret - "I feared that he was coming"

Because 'timui' is in the perfect tense, 'veniat' becomes 'veniret' (imperfect subjunctive) following the sequence of tenses.

When putting a fear clause into the negative it is usual for 'ne' to become 'ut'. Nothing else changes

e.g timeo ut veniat - "I fear that he is not coming"

timui ut veniret "I feared that he was not coming"

But if you want to translate a phrase such as "I am afraid to fight him" you use the infinitve, just as in English. So:

timeo eum pugnare

The main thing to learn here is the sequence of tenses.

If the verb of fearing is in the present, future or future perfect tense then the verb following 'ne/ut' is in the present /perfect subjunctive.

If the verb of fearing is in the imperfect, perfect or pluperfect tense then the verb following 'ne/ut' is in the imperfect / pluperfect subjunctive.