Learn Hardcore Russian: Я полностью понимаю грамматику.

Russian does not have definite or indefinite articles (no equivalents of a/an or the). Nouns stand alone without articles. You simply say грамматику for “grammar” and rely on context to tell whether it’s specific or general.

What case is the word грамматику in, and why is it that case?

Грамматику is in the accusative case. In Russian, the direct object of a transitive verb (someone/something that the verb acts upon) takes the accusative. Here, понимаю (“I understand”) acts upon грамматику, so we use the feminine singular accusative form of грамматика.

What part of speech is полностью, and why is it used here?

Полностью is an adverb meaning “completely” or “fully.” It modifies the verb понимаю by specifying the degree of understanding. Adverbs in Russian often end in -о or -е and answer questions like “how?” or “to what extent?”

Why use the imperfective verb понимаю instead of a perfective like понял or понять?

Russian verbs come in imperfective and perfective aspects.

Imperfective понимать/понимаю expresses ongoing, habitual, or general actions: “I (habitually or right now) understand.”
Perfective понять/понял indicates a completed action: “I have understood.”
Since you’re stating a general ability or current ongoing understanding, you use the imperfective понимаю.

Is the word order fixed? Could I say Я понимаю грамматику полностью or Полностью я понимаю грамматику?

Word order in Russian is relatively flexible, thanks to its case system.

Я понимаю грамматику полностью is perfectly natural and shifts emphasis slightly onto “completely.”
Полностью я понимаю грамматику sounds a bit more literary or emphatic—putting Полностью at the front stresses “fully.”
All these variants are grammatically correct; the choice depends on what you want to highlight.

Where do you put the stress in полностью, понимаю, and грамматику?

по́лностью – stress on the first о
понима́ю – stress on а́ю (по-ни-МА́-ю)
грамма́тику – stress on а́ (грам-ма́-ти-ку)
Knowing stress positions is crucial for correct pronunciation.

Can I replace грамматику with a pronoun, for example Я полностью её понимаю?

Yes. Её (“it,” referring to grammar) stands in for грамматику and agrees in gender (feminine). The sentence Я полностью её понимаю is perfectly natural and often used once the noun has been introduced.

Why not say Я понимаю грамматику до конца for “I fully understand grammar”?

You could say до конца (“to the end”), but it has a more literal sense: “I understand grammar up to the end.” It often implies completion of a course or book rather than a general mastery. Полностью понимаю is the standard way to express “I fully/completely understand.”