Если я ошибусь, это не страшно.

Ошибусь is the 1st person singular future form of the verb ошибиться.

So:

• я ошибусь = I will make a mistake / I’ll be mistaken

Because ошибиться is a perfective verb, it forms the future with a simple future form, not with буду.

Compare:

• ошибиться → я ошибусь
• делать → я буду делать

In this sentence, the speaker means a possible future mistake, so ошибусь fits naturally.

That is a very important difference between English and Russian.

In Russian, after если referring to the future, you normally do use a future form:

• Если я ошибусь, ... = If I make a mistake, ...

English prefers the present after if, but Russian does not follow that rule.

More examples:

• Если он придёт, мы начнём. = If he comes, we’ll start.
• Если будет дождь, останемся дома. = If it rains, we’ll stay home.

So Если я ошибусь is exactly what Russian grammar expects.

Russian uses aspect to show how an action is viewed.

Here, ошибусь comes from ошибиться, which is perfective. Perfective is used because the sentence refers to one possible completed event:

• If I make a mistake = one mistake, one occasion

That is why perfective sounds natural here.

If you used the imperfective, the meaning would change more toward:

• a general tendency
• repeated action
• an ongoing process

So in this sentence, perfective is the best choice because the speaker means if it happens once.

You can, but it means something different.

• Если я ошибусь = If I make a mistake / If I turn out to be wrong in the future
• Если я ошибаюсь = If I am mistaken right now

So:

• Если я ошибусь, это не страшно = future possibility
• Если я ошибаюсь, это не страшно = if I’m wrong now, that’s not a big deal

The original sentence is about a future situation, so ошибусь is the right form.

Это refers to the whole situation: the fact that I might make a mistake.

So the structure is basically:

• Если я ошибусь, это не страшно
• If I make a mistake, that is not страшно

In natural English, we usually just say:

• If I make a mistake, it’s not a big deal
• If I make a mistake, that’s not a problem

Russian often uses это this way to point back to the whole previous idea.

You can leave it out in some contexts:

• Если я ошибусь, не страшно.

This is understandable and quite natural in speech.

But это makes the sentence feel a bit more complete and neutral:

• Если я ошибусь, это не страшно.

So:

• with это = slightly fuller, more explicit
• without это = more conversational, compact

Both are possible.

Because страшно here is not describing a noun.

It is being used in an impersonal/predicative way, meaning something like:

• it is scary
• it is terrible
• it is a big deal

In Russian, words like this often appear in the neuter form:

• интересно
• важно
• трудно
• страшно

So:

• Это не страшно = It’s not terrible / It’s not a big deal

If it were describing a noun, then it would have to agree with that noun:

• страшная ошибка = a terrible mistake
• страшный фильм = a scary movie

But here it is not modifying a noun, so страшно is correct.

Because in Russian, the verb to be is usually omitted in the present tense.

So:

• Это не страшно literally corresponds to This not страшно
• but naturally it means This is not a big deal

In the past or future, Russian does use forms of быть:

• Это было не страшно. = It was not scary / not a big deal.
• Это будет не страшно. = It will not be scary / not a big deal.

But in the present tense, the copula is normally absent.

It is often more idiomatic than the literal translation suggests.

Depending on context, не страшно can mean:

• it’s not scary
• it’s not terrible
• it’s not a problem
• it’s no big deal
• that’s okay

In your sentence, the most natural English idea is:

• If I make a mistake, it’s not a big deal.

So even though страшно is related to fear and something scary, the phrase не страшно is very commonly used to reassure someone.

Because Если я ошибусь is a subordinate clause introduced by если.

Russian normally separates such clauses with a comma:

• Если я ошибусь, это не страшно.

This is standard punctuation.

English also often uses a comma when the if-clause comes first:

• If I make a mistake, it’s not a big deal.

So the punctuation here is very straightforward.

Yes, but there are some differences in style and usage.

• Если я ошибусь = very natural, compact, everyday
• Если я сделаю ошибку = understandable, but less idiomatic than English make a mistake
• Если я совершу ошибку = correct, but more formal/bookish

In many everyday situations, Russian prefers the verb ошибиться rather than a literal equivalent of make a mistake.

So for natural spoken Russian, Если я ошибусь is usually the best choice.

Yes. Russian word order is flexible.

You can also say:

• Это не страшно, если я ошибусь.

The core meaning stays the same, but the focus changes a little.

• Если я ошибусь, это не страшно.
  Focus first on the condition: if I make a mistake
• Это не страшно, если я ошибусь.
  Focus first on the reassurance: that’s not a big deal

The original version is very natural because it presents the possible situation first, then the reaction to it.

The stress is on the last syllable:

• ошибу́сь

A rough pronunciation guide:

• a-shi-BOOS'

And the whole sentence sounds roughly like:

• YE-sli ya a-shi-BOOS', EH-ta nye STRASH-na

Word stress:

• Е́сли
• ошибу́сь
• э́то
• стра́шно

Stress is important in Russian, so it is worth learning ошибу́сь with the stress included.