Я не учу весь текст наизусть, но хочу знать наизусть хотя бы несколько важных фраз.

They are related, but they are not the same.

• учить / учу = to learn / to study / to memorize
  This focuses on the process.
• знать = to know
  This focuses on the resulting state.

So:

• Я не учу весь текст наизусть = I am not memorizing the whole text
• но хочу знать наизусть... = but I want to know ... by heart

This is very natural in Russian: first, the speaker says what they are not trying to memorize, and then says what they do want to know.

Наизусть means by heart, from memory, or by memorization.

It is used when someone knows something so well that they can say it without reading it.

Examples:

• учить наизусть = to learn by heart
• знать наизусть = to know by heart
• рассказать наизусть = to recite from memory

So in this sentence, наизусть shows that the speaker is talking about memorizing exact wording, not just understanding the general meaning.

Yes, it could be said only once, but repeating it is completely normal and makes the contrast clearer.

The sentence is:

• Я не учу весь текст наизусть
• но хочу знать наизусть хотя бы несколько важных фраз

The speaker wants to emphasize memorization in both parts:

• not memorizing the whole text,
• but wanting to know at least some phrases by heart.

You could also say:

• Я не учу весь текст наизусть, но хочу хотя бы несколько важных фраз знать наизусть.

But the original version sounds more natural and balanced.

Because текст is the direct object of учу, and in this sentence it is in the accusative case.

• весь agrees with текст
• текст is masculine singular inanimate
• for masculine inanimate nouns, the accusative looks the same as the nominative

So:

• nominative: весь текст
• accusative: весь текст

That is why the form does not change.

If the noun were feminine, you would see the accusative more clearly:

• вся книга = the whole book
• учу всю книгу = I am learning the whole book

Because несколько usually requires the genitive plural.

So:

• фраза = phrase
• genitive plural = фраз

And the adjective must match:

• важный = important
• genitive plural = важных

That gives:

• несколько важных фраз = several important phrases

This is a very common Russian pattern:

• несколько книг = several books
• несколько интересных книг = several interesting books
• несколько слов = several words

Хотя бы means at least.

It shows that the speaker is lowering the goal to a minimum acceptable amount.

So:

• хочу знать наизусть хотя бы несколько важных фраз
  = I want to know at least a few important phrases by heart

The feeling is:

• not the whole text,
• but at minimum, some important phrases.

It is a very common expression in Russian.

Examples:

• Скажи хотя бы одно слово. = Say at least one word.
• Приди хотя бы ненадолго. = Come at least for a little while.

Учу is imperfective because it describes an ongoing process or general activity.

• учить = to be learning / studying / memorizing
• выучить = to learn completely / memorize successfully

In this sentence, the speaker is talking about what they are doing or not doing as a method, not about a completed result.

So:

• Я не учу весь текст наизусть = I’m not memorizing the whole text

If you said:

• Я не выучу весь текст наизусть

that would mean something more like:

• I will not manage to learn the whole text by heart
• or I’m not going to learn the whole text by heart

That is a different idea.

Yes, it could be omitted in many contexts, because the verb form already shows the subject:

• учу = I learn / I am learning
• хочу = I want

So Russian often drops subject pronouns when they are obvious.

However, Я is still very common, especially:

• at the beginning of a sentence,
• when introducing a personal opinion,
• when creating a contrast.

Here, Я sounds natural because the speaker is clearly stating their own approach:

• I’m not doing X, but I want Y

Without Я, the sentence would still work:

• Не учу весь текст наизусть, но хочу знать наизусть хотя бы несколько важных фраз.

But the original version is slightly clearer and more neutral.

Но means but.

It connects two contrasting ideas:

1. Я не учу весь текст наизусть
   = I’m not memorizing the whole text

2. но хочу знать наизусть хотя бы несколько важных фраз
   = but I want to know at least a few important phrases by heart

So the contrast is:

• not everything
• but at least something important

This is exactly the same basic logic as English but.

Yes, Russian word order is flexible, but the original order is a very natural, neutral one.

The sentence places things in a clear progression:

• хочу = I want
• знать наизусть = to know by heart
• хотя бы несколько важных фраз = at least a few important phrases

Other orders are possible, for example:

• хочу хотя бы несколько важных фраз знать наизусть
• хочу знать хотя бы несколько важных фраз наизусть

These are understandable, but the original version sounds smooth and standard.

In Russian, changing word order often changes:

• emphasis,
• rhythm,
• what sounds most natural in context.

Yes. Фраза means phrase, so it suggests the speaker wants to memorize useful chunks of language, not the entire text.

Compare:

• слова = words
  focus on individual vocabulary items
• фразы = phrases
  focus on useful expressions
• предложения = sentences
  focus on complete sentences

So несколько важных фраз sounds very practical, especially for language learning: the speaker does not want to memorize everything, only some key expressions.

In this sentence, весь текст is best understood as the whole text or the entire text.

That is slightly different from English all in some contexts.

• весь текст = the whole text
• все слова = all the words

So весь often means whole / entire with a singular noun.

Examples:

• весь день = the whole day
• вся книга = the whole book
• всё письмо = the whole letter

Here, Я не учу весь текст наизусть means the speaker is not trying to memorize the text in full.