188. Verbs of the 3rd Conjugation in -iō have certain forms of the Present stem like the 4th conjugation. They lose the i of the stem before a consonant and also before ĭ, ī, and ĕ (except in the Future, the Participle, the Gerund, and the Gerundive).1 Verbs of this class are conjugated as follows.
1. This is a practical working rule. The actual explanation of the forms of such verbs is not fully understood.