Сколько ни повторяй новые слова, без контекста их всё равно трудно понять.

Here сколько does not ask a question. In the pattern сколько ни + verb, it means:

• no matter how much
• however many times
• even if you repeat it over and over

So Сколько ни повторяй новые слова... means something like:

• No matter how much you repeat new words...
• However many times you repeat new words...

This is a very common Russian concessive pattern:

• что ни делай = no matter what you do
• как ни старайся = no matter how hard you try
• сколько ни читай = no matter how much you read

Grammatically, повторяй is the imperative form of повторять. But in this kind of construction, it often does not function as a real command.

Instead, Russian uses the imperative here in a generalized sense:

• Сколько ни повторяй... = however much one repeats...
• Как ни крути... = however you look at it / no matter how you spin it...

So it is not literally telling someone Repeat!
It is more like a general statement about what happens even if you do that action many times.

Because this is a fixed concessive pattern: interrogative word + ни + verb.

Examples:

• что ни говори = no matter what you say
• где ни ищи = no matter where you look
• сколько ни повторяй = no matter how much you repeat

In these expressions, ни is standard and does not simply mean ordinary negation. It helps create the sense of no matter..., however..., even if....

So сколько не повторяй would usually not be the normal form for this meaning.

Новые слова is in the accusative plural, because it is the direct object of повторяй:

• повторять что? → новые слова

For inanimate plural nouns, the accusative plural is often the same as the nominative plural:

• новые слова = nominative plural
• новые слова = accusative plural

So the form does not change, but the function does: here it is the object of the verb.

Because их refers back to новые слова and acts as the object of понять:

• без контекста их всё равно трудно понять
• without context, it is still hard to understand them

Russian often repeats the object with a pronoun in the second clause rather than leaving it implied.

Structure:

• first clause: повторяй новые слова
• second clause: их трудно понять

So их = the words / them

Here их is in the accusative plural, because it is the direct object of понять:

• понять кого? что? → их

Since их has the same form in both genitive plural and accusative plural, you identify the case from the syntax and the verb.

Here it is clearly the thing being understood, so it is accusative.

Всё равно here means:

• all the same
• anyway
• still
• nevertheless

So the idea is:

• Even if you repeat new words many times, without context it is still hard to understand them.

It emphasizes that repeating alone does not solve the problem.

Compare:

• Я всё равно пойду. = I’ll go anyway / all the same.
• Это всё равно трудно. = It’s still difficult.

This is an impersonal construction:

• трудно понять = it is difficult to understand

Here трудно is an adverb/predicative word, not an adjective modifying a noun. Russian often uses this pattern:

• трудно понять = hard to understand
• легко запомнить = easy to remember
• невозможно объяснить = impossible to explain

There is no grammatical subject like English it. Russian simply says:

• Их трудно понять. Literally: Them hard to understand. Naturally: It is hard to understand them.

As for понимаются, that would be a completely different structure and is not natural here.

Russian often uses the perfective infinitive after words like трудно, легко, невозможно, when talking about successfully achieving a result.

• понять = to understand, to grasp
• понимать = to understand in an ongoing/general sense

Here the idea is about reaching understanding:

• их трудно понять = it’s hard to understand/grasp them

That makes понять the natural choice.

You could sometimes see понимать in other contexts, but трудно понять is the standard phrasing here.

Because the preposition без always takes the genitive:

• без чего? → без контекста
• без книги
• без времени
• без друзей

So:

• контекст = nominative
• без контекста = genitive after без

This is just a normal prepositional case rule.

It gives the condition or circumstance under which the words are hard to understand:

• without context

So the sentence contrasts two things:

1. repeating new words
2. understanding them without context

The point is that repetition alone is not enough; context matters.

The word order is natural and emphasizes the condition first:

• без контекста → first, setting the frame
• их → the thing being understood
• всё равно → still / anyway
• трудно понять → the main predicate

A more neutral English-like breakdown would be:

• Без контекста их трудно понять.
• Их без контекста трудно понять.

Both are possible, but the original order sounds smooth and highlights without context early.

Russian word order is flexible, but changes emphasis more than basic meaning.

Yes, a version with бы is also possible in Russian, for example:

• Сколько бы ни повторял новые слова, без контекста их всё равно трудно понять.

This sounds a bit more explicitly conditional/hypothetical:
no matter how much you might repeat...

The version in your sentence, Сколько ни повторяй..., is very natural and compact. Russian often uses this shorter imperative-based pattern in general statements.

Both express roughly the same idea, but the original is especially idiomatic.

There is no explicit subject, and that is normal in Russian.

In Сколько ни повторяй, the imperative form literally looks like you repeat, but in meaning it is generalized:

• you in a generic sense
• one
• anyone

In их всё равно трудно понять, the construction is impersonal:

• literally no subject is stated
• naturally in English: it is still hard to understand them

So overall the sentence is a general truth, not a comment about one specific person.