Если я сейчас не доучу это правило, завтра на уроке мне будет трудно.

Доучу comes from доучить. The prefix до- often adds the idea of finishing something that is already partly done.

So here не доучу means something like:

• I won’t finish learning
• I won’t finish studying
• I won’t fully learn the rest of it

That is a little different from выучу, which usually means learn/master completely as a whole.

So:

• выучить правило = to learn the rule
• доучить правило = to finish learning the rule

This sentence suggests the speaker has already started learning the rule and still has some left to finish.

That is a very common question, because Russian and English work differently here.

In Russian, after если for a real future condition, you can use a future form:

• Если я не доучу...

Because доучить is a perfective verb, its present-looking forms actually have future meaning. So доучу means I will finish.

So the structure is normal Russian:

• Если я не доучу это правило... = If I don’t finish learning this rule...

Even though English avoids will after if, Russian does not follow that same rule.

Yes, сейчас does mean now, but it can be understood a bit more broadly than just this exact second.

In this sentence it means something like:

• now / right now
• at this point
• before tomorrow
• during the current study time

So Если я сейчас не доучу это правило... means:

• If I don’t finish learning this rule now...

The action of finishing is still future from the speaker’s point of view, but сейчас tells you the relevant time frame is now, not later.

Правило is the direct object of доучу, so it is in the accusative case.

However, правило is:

• neuter
• singular
• inanimate

For neuter inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: это правило
• accusative: это правило

The form does not change, but the case function does.

Because the first part is a subordinate clause introduced by если:

• Если я сейчас не доучу это правило = subordinate clause
• завтра на уроке мне будет трудно = main clause

In Russian, clauses like this are normally separated by a comma.

So the comma is required:

• Если ..., ...

Russian normally says на уроке for in class / during the lesson / at the lesson.

This is just the usual preposition used with many events or organized activities:

• на уроке = in class
• на лекции = at the lecture
• на занятии = at the lesson/session
• на собрании = at the meeting

After на in this meaning, урок is in the prepositional case:

• урок → на уроке

So на уроке is the natural idiomatic choice.

Russian often uses an impersonal construction for feelings, states, and levels of difficulty.

So:

• мне будет трудно

literally means something like:

• to me it will be difficult

This is a very common Russian pattern.

The person experiencing the difficulty is put in the dative case:

• мне = to me

This sounds more natural in Russian than trying to build the sentence around я.

English often says:

• I will have difficulty
• It will be hard for me

Russian often prefers:

• мне будет трудно
• мне будет сложно
• мне будет тяжело

Because трудно here is not an adjective describing a noun.

It is a predicative word meaning:

• difficult
• hard

It describes the whole situation, not a noun.

Compare:

• трудный урок = a difficult lesson
  Here трудный is an adjective modifying урок.

• мне будет трудно = it will be hard for me
  Here трудно is a predicative word, so it does not agree with a noun.

That is why you do not use трудный in this sentence.

Nothing is grammatically missing. Russian often leaves the action understood from context.

So:

• мне будет трудно

can stand by itself if the listener already understands what will be hard — for example:

• to follow the lesson
• to understand the material
• to answer the teacher
• to do the exercises

If you want to be more specific, you can add an infinitive:

• мне будет трудно понять тему
• мне будет трудно отвечать
• мне будет трудно работать на уроке

But in your sentence, leaving it unsaid is completely natural.

Yes. Russian word order is fairly flexible, and changing it usually changes emphasis, not the basic meaning.

For example, these are all possible:

• Если я сейчас не доучу это правило, завтра на уроке мне будет трудно.
• Если я сейчас не доучу это правило, завтра мне будет трудно на уроке.
• Завтра на уроке мне будет трудно, если я сейчас не доучу это правило.

The original sentence sounds natural. It presents the condition first, then the consequence.

It sounds like a real, likely future situation, not a distant or unreal hypothetical.

The speaker means something like:

• If I don’t finish this now, tomorrow I’ll have trouble in class.

Russian often uses если + future-oriented forms for this kind of practical, real condition.

If the speaker wanted to express something more hypothetical or unreal, Russian would often use a different pattern involving бы.

So this sentence is best understood as a real warning / real consequence.

Yes, all three are possible, but the nuance changes a little.

• трудно = hard, difficult
• сложно = difficult, complicated
• тяжело = hard, heavy, difficult in a more burdensome sense

So:

• мне будет трудно = neutral, very natural here
• мне будет сложно = also natural, maybe slightly more like it will be complicated
• мне будет тяжело = possible, but often feels more emotionally or physically heavy

For this sentence, трудно is probably the most neutral and standard choice.