This is the 878th most frequent Czech word.
vysvětlil
Explained.
In this sentence, 'vysvětlil' is in the past tense and third person singular, indicating an action completed by a male subject, the teacher, towards the students.
Učitel vysvětlil žákům nový koncept.
The teacher explained the new concept to the students.
Here, 'vysvětlil' is again in the past tense and third person singular, referring to Petr providing reasoning for his tardiness.
Petr vysvětlil, proč přišel pozdě na schůzku.
Petr explained why he was late to the meeting.
In this instance, 'vysvětlil jsem' is in the past tense and first person singular, indicating that the speaker performed the explained action.