gakusei ha kokuban wo mimasu.

• 学生 (がくせい) – noun, “student”
• は – topic‐marking particle (sets “学生” as the topic)
• 黒板 (こくばん) – noun, “blackboard”
• を – direct‐object particle (marks “黒板” as what is being looked at)
• 見ます (みます) – verb, polite present form of 見る, “to look/see”

• は marks the topic (“As for the student…”), focusing on what the student does.
• が would mark 学生 as the grammatical subject in a neutral declaration (“The student does X”), often introducing new info.
• Using は implies we’re talking about a known or previously mentioned student.

Yes. 見る is the dictionary (plain) form.
• 学生は黒板を見ます。 – polite/formal speech (e.g. in class, talking to a teacher).
• 学生は黒板を見る。 – plain style (e.g. diary, talking with friends).
The meaning stays “Students look at the blackboard,” but the tone shifts.

Japanese does not have articles like “a” or “the.” Context does the work.
• If listeners know which board you mean, it’s understood as “the blackboard.”
• If you want to emphasize indefiniteness (“a blackboard”), you might add ある: 学生はある黒板を見ます。 (“The students look at a (some) blackboard.”)

Yes. Japanese often drops topics or subjects when they’re understood.
• （学生は）黒板を見ます。 – perfectly natural if it’s already clear you’re talking about students.

• 学生 typically refers to college/university students.
• 生徒 (せいと) usually means K–12 students (elementary, middle, high school).
Choose based on educational level. In a classroom context for younger kids, 生徒は黒板を見ます might feel more appropriate.