The sentence is an example of "present indefinite sentence" in the affirmative version which suggests the use of the first form of the verb i.e. "present participle" form of the verb. The statement exemplifies a habitual action of the speaker which is written with the use of present participle form of the verb. "I <u>ring</u> the bell when it is time for the doors to open". "ring"(present participle) here is the verb that draws the answer of the question.
