Я сказала подруге, что шпаргалка вряд ли поможет, если не понимаешь тему.

Because the verb is in the past tense, and in Russian past-tense verbs agree with the speaker’s gender in the singular.

• Я сказал = I said (male speaker)
• Я сказала = I said (female speaker)

So this sentence tells you that the speaker is female.

Because after сказать (to say / tell), the person you tell something to usually goes in the dative case.

• сказать кому? = to tell whom?

So:

• подруга = female friend (nominative)
• подруге = to a female friend (dative)

In this sentence:

• Я сказала подруге = I told my friend / I said to my friend

This is very common in Russian:

• Я сказал брату
• Она сказала маме
• Мы сказали учителю

Russian often leaves out possessive words like my, your, his, etc. when the meaning is obvious from context.

So:

• Я сказала подруге literally looks like I said to friend
• but naturally it usually means I told my friend

If you want to be more explicit, you can say:

• Я сказала своей подруге

That also means I told my friend, but it sounds a bit more specific or emphatic.

Вряд ли means hardly, unlikely, or probably not.

In this sentence:

• шпаргалка вряд ли поможет = a cheat sheet probably won’t help / is unlikely to help

It expresses doubt that something will be useful.

A few examples:

• Он вряд ли придёт. = He probably won’t come.
• Это вряд ли возможно. = That’s unlikely to be possible.

It is a very common expression, and learners should treat it almost like a fixed phrase.

Yes. Поможет is the future tense of the perfective verb помочь (to help).

That fits the meaning well here, because the sentence is about whether the cheat sheet will help in that situation.

• помогать = imperfective, more about process or repeated helping
• помочь = perfective, more about a single result: to help successfully

So:

• шпаргалка поможет = the cheat sheet will help
• шпаргалка вряд ли поможет = the cheat sheet probably won’t help

The perfective verb is natural because the focus is on the result, not on an ongoing process.

After если (if), Russian often uses the present tense to talk about a future condition.

So even though English might think about the future, Russian normally says:

• если не понимаешь тему
  literally: if you don’t understand the topic

This can mean:

• if you don’t understand the topic (in general)
• if you don’t understand the topic when the situation comes up

Russian does not usually use a future form after если in this kind of sentence.

Compare:

• Если придёшь, позвони. = If you come, call me.
• Если не понимаешь тему, шпаргалка не поможет. = If you don’t understand the topic, a cheat sheet won’t help.

In Russian, the subject pronoun is often omitted because the verb ending already shows the person and number.

• понимаешь = you understand (singular / informal)

So не понимаешь already means you don’t understand.

Here it is used in a general sense, like English if you don’t understand the topic meaning if a person doesn’t understand the topic.

Russian does this a lot:

• Когда читаешь быстро, легко ошибиться. = When you read quickly, it’s easy to make mistakes.

It does not necessarily mean the speaker is addressing one specific person informally. It can be a general you.

Because тема is the direct object of понимать, so it goes in the accusative case.

• тема = nominative
• тему = accusative

You understand what?

• понимаешь тему

Other examples:

• понимать вопрос = to understand the question
• понимать проблему = to understand the problem

Since тема is a feminine noun ending in -а, its accusative singular becomes -у:

• тема → тему

Шпаргалка is an informal everyday word meaning something like:

• cheat sheet
• crib sheet
• exam note used for cheating

It usually refers to a small note or hidden aid used in school or exams.

The tone is conversational, not formal. In a formal context, you might explain it more explicitly, but in normal speech шпаргалка is very common.

In this sentence, it suggests: knowing the material matters more than relying on a cheat sheet.

Because both что and если introduce subordinate clauses, and Russian separates those clauses with commas.

Structure:

• Я сказала подруге,
  main clause

• что шпаргалка вряд ли поможет,
  subordinate clause introduced by что

• если не понимаешь тему.
  subordinate clause introduced by если

So the commas are standard Russian punctuation.

Here что means that and introduces reported speech or reported content.

• Я сказала подруге, что... = I told my friend that...

This is one of the most common uses of что.

Compare:

• Он сказал, что устал. = He said that he was tired.
• Я знаю, что это правда. = I know that it is true.

So in this sentence, что connects I told my friend with what I told her.

Not with the same meaning.

• понимать тему = to understand the topic
• не понимать тему = not to understand the topic

Here the direct object stays in the accusative: тему.

Russian sometimes uses the genitive after negation, but with verbs like понимать, the accusative is normally the natural choice here.

So:

• если не понимаешь тему is the normal version

Yes, Russian word order is flexible, but changing it changes the emphasis.

The given sentence is neutral and natural:

• Я сказала подруге, что шпаргалка вряд ли поможет, если не понимаешь тему.

You could move things around for emphasis, but not every variation sounds equally natural. For example:

• Я подруге сказала, что...
  puts more emphasis on to my friend

• Если не понимаешь тему, шпаргалка вряд ли поможет.
  starts with the condition and sounds very natural too

That version is especially common because it presents the condition first:

• If you don’t understand the topic, a cheat sheet probably won’t help.

So yes, word order can change, but the original is a normal, neutral sentence.