Learn Hardcore Russian: Учитель сидит в классе.

The preposition в here indicates location, so it requires the noun класс to be in the prepositional case—in this case, классе. This usage shows where the action is taking place (i.e., "in the classroom").

What is the tense and aspect of the verb сидит?

The verb сидит is in the present tense, third person singular form of сидеть. Since сидеть is imperfective, it describes an ongoing or habitual action—in this sentence, it tells us that the teacher is currently in the state of sitting.

Why are there no articles like the before учитель and классе?

Russian does not use articles such as a or the. The specificity (or generality) of nouns like учитель and классе is understood from context rather than from explicit articles, which is why they appear without any additional words.

What is the sentence structure of Учитель сидит в классе, and can the order be changed?

This sentence follows a typical Subject-Verb-Location pattern: учитель (subject), сидит (verb), в классе (location). Although Russian allows for flexible word order (which can be rearranged for emphasis), this standard structure clearly identifies who is performing the action and where it occurs.