Если я долго не ем, я начинаю слабеть.

Если literally means if, but in sentences like this it often translates naturally as when in English, especially when talking about something that regularly happens.

So:

• Если я долго не ем, я начинаю слабеть. = If I don’t eat for a long time, I start to get weak = very naturally in English, When I don’t eat for a long time, I start to get weak

Russian often uses если in general cause-and-effect statements where English might prefer when.

Because the sentence describes a general repeated situation, not one specific future event.

• ем = I eat / I am eating
• не ем = I do not eat / I am not eating

In Russian, the present tense is commonly used for habits, repeated situations, and general truths:

• Если я долго не ем, я начинаю слабеть. = If/when I go a long time without eating, I start getting weak.

It is not about one future occasion, but about what generally happens.

Долго means for a long time.

So:

• я долго не ем = I don’t eat for a long time
• more natural English: I go a long time without eating

It modifies the verb ем and tells you the duration of the not-eating.

A learner should notice that Russian often says this more directly than English. English often prefers:

• If I don’t eat for a long time...
• If I go a long time without eating...

Russian commonly expresses this idea with simple negation:

• долго не ем = I don’t eat for a long time

English often rephrases this as:

• I go a long time without eating

But Russian does not need a special without eating structure here. The straightforward не + verb is normal and natural.

Ем is the 1st person singular present tense of the verb есть (to eat).

Conjugation of есть in the present:

• я ем = I eat
• ты ешь = you eat
• он / она ест = he/she eats
• мы едим = we eat
• вы едите = you (plural/formal) eat
• они едят = they eat

So не ем simply means I don’t eat.

Yes, it could be omitted, because the verb ending already shows the subject:

• Если я долго не ем, начинаю слабеть.

This is grammatically possible.

But Russian often keeps the pronoun for clarity, rhythm, or emphasis, especially when there are two clauses:

• Если я долго не ем, я начинаю слабеть.

Including the second я makes the sentence feel a bit clearer and more balanced. It is very normal.

This is a very common Russian pattern:

• начинать / начать + infinitive = to begin / start doing something

So:

• начинаю = I begin / I start
• слабеть = to grow weak / to become weaker

Together:

• я начинаю слабеть = I start to get weak

Russian often uses this structure exactly like English:

• начинаю работать = I start working
• начинаю понимать = I’m starting to understand

Because слабеть means to become weak / to grow weaker, not simply to be weak.

Compare:

• быть слабым = to be weak (a state)
• слабеть = to become weak (a change/process)

In this sentence, the speaker means that after not eating for a long time, they start becoming weak, so слабеть is the better choice.

Слабеть is imperfective.

That fits because начинать normally takes an infinitive describing a process or ongoing action/state change:

• начинать слабеть = to begin getting weak

The focus is on the start of the process, not on a completed result.

A perfective partner such as ослабеть means something more like to become weak / to weaken completely or to a noticeable result, but after начинать, Russian usually prefers the imperfective infinitive:

• начинать слабеть = natural
• начинать ослабеть = not normal

It could, depending on context, but I start to get weak is closer to the Russian wording because of начинаю.

• слабею alone could mean I get weak / I become weak
• начинаю слабеть more specifically means I start getting weak

So if you want to reflect the Russian more accurately, keep the idea of starting.

Yes, Russian word order is flexible. This version is the most neutral and natural.

Possible variations include:

• Если я долго не ем, я начинаю слабеть.
• Я начинаю слабеть, если я долго не ем.

Both are correct.

The version with если first is especially common because it presents the condition first and then the result.

Within the clause, долго can sometimes move too, but the original order is the clearest for learners:

• я долго не ем = neutral and natural

Я is in the nominative case, because it is the subject of both verbs:

• я не ем = I do not eat
• я начинаю слабеть = I start getting weak

There are no other noun cases in this sentence because there is no direct object or prepositional phrase requiring them.

Yes, it is completely natural and something a person could really say.

It sounds neutral and standard. In everyday speech, a native speaker might also say similar things such as:

• Если долго не есть, начинаешь слабеть.
  = If you don’t eat for a long time, you start to get weak.

• Когда я долго не ем, я слабею.
  = When I don’t eat for a long time, I get weak.

But your original sentence is perfectly normal and clear.