Verbs in -aître and -oître
Verbs ending in -aître and -oître are few in number. The following verbs and their derivatives can be mentioned: paraître (to appear), connaître (to know), naître (to be born), croître (to grow). However, they are remarkable because the rule is relatively simple, all the more so as they are quite frequent verbs.
Verbs in -aître and -oître retain their circumflex accent when the i of the radical is placed before a t.
je connais / il connaît
je connaissais / je connaîtrai
The 1990 reform
In the spelling changes of the 1990 reform, it is proposed to drop this circumflex accent. The circumflex accent of the i in front of the t can be removed except in the case of the simple past tense: "vous naquîtes" but "il nait" instead of "il naît".
Verb croître
Croître (to grow) has a circumflex accent in all forms where it could be mistaken for believing. This means that it does not concern the verbs "accroître" (to increase), "décroître" (to decrease) and "recroître" (to re-grow) which follow the rule described above. In classical spelling, however, "croître" (to grow) has the peculiarity of taking a circumflex accent on its past participle in the masculine past tense only. One then writes crû, crue, crus and crues. Since 1990, it has been recommended to keep this circumflex accent in all forms, including the forms of the past participle.
Despite this rule, some tenses such as the imperfect subjunctive of the verbs "croître" (to grow) and "croire" (to believe) are identical and the context must be used to differentiate them.
je croîs (croître) / je crois (croire)
qu'il crût (croître/croire)
Verbs clore, gésir and plaire
These three verbs take a circumflex accent in front of the t: il clôt, il gît, il plaît. So please pay attention to the writing of please : s'il vous plaît. However, this is not a general rule since a verb like taire does not have a circumflex accent in front of the t.
When the spelling was revised in 1990, the Académie française recommended dropping the circumflex accent and simply writing: il plait.