Final answer:
The blanks in the sentence should be filled with 'clean' for the regular habit in the present, and 'cleaned' for the specific action that happened yesterday, in the past.
Step-by-step explanation:
The student's question involves filling in the blanks using the correct tense of the verb clean. The sentence should read: I usually clean my teeth early in the morning, but yesterday I cleaned at night.
To answer this question, you need to recognize the tense required for each blank in the sentence. In the first part of the sentence, we are discussing a habitual action, something the student does regularly. Therefore, the simple present tense is needed: clean. In the second part, the sentence refers to a specific action completed in the past, which is why the simple past tense, cleaned, is correct.