What are the two criteria for a French verb to be considered a regular -IR verb?

The verb must end in -IR in its infinitive form and have a present participle ending in -issant.

What is the significance of the '-issant' ending in the present participle of a French verb?

It indicates that the verb belongs to the second group of regular -IR verbs.

Drop the -IR ending from the infinitive and add the appropriate endings: -is, -is, -it, -issons, -issez, -issent.

Because irregular -IR verbs do not follow the same conjugation pattern as regular -IR verbs.

I can use some common French -IR verbs in the present tense that don't follow the regular pattern.

1
Regular French -IR verbs meet two criteria: they end in -IR in their infinitive form and have a present participle ending in -issant.
2
To conjugate regular -IR verbs in the present tense, drop the -IR ending and add the appropriate endings, such as -is, -is, -it, -issons, -issez, -issent.
3
Some -IR verbs belong to the third group and are irregular, so you must learn them individually, as they do not follow the regular -IR verb conjugation pattern.