If statements (a.k.a. conditionals) refer to sentences that go something like “If some action occurs …”. The negative form of this is something like “If some action doesn’t occur …”.
For verbs, the past tense stem is the part of the past tense verb to which the verb ending is added. See Verbs - Past Tense for details.
To turn a phrase into an if statement, add -ஆல் to the past tense stem of the verb
To turn a phrase into a negative if statement, replace the final -அ from the infinitive with -ஆவிட்டால்