אני מגיעה מוקדם, כלומר לפני שכולם נכנסים לחדר.

Because the speaker is female singular.

In Hebrew present tense, many verb forms agree with the subject’s gender and number:

• אני מגיע = I arrive / I’m arriving (male speaker)
• אני מגיעה = I arrive / I’m arriving (female speaker)

So this sentence is being said by a woman or girl.

It is the present tense form of the verb.

In Hebrew, the present tense can cover several ideas that English separates:

• I arrive early
• I am arriving early
• sometimes even a planned future, depending on context

So אני מגיעה מוקדם can mean either a general habit or something happening now / in the near future.

Because מוקדם here is being used adverbially, meaning early.

When Hebrew uses an adjective like this to describe how something happens, it often stays in the masculine singular form, even if the subject is feminine.

So:

• אני מגיעה מוקדם = I arrive early

But if it directly described a feminine noun, it would agree:

• הגעה מוקדמת = an early arrival

That is a very common point for learners.

כלומר is a connector meaning:

• that is
• that is to say
• in other words

It introduces a clarification or a more precise explanation.

So the sentence first says I arrive early, and then כלומר explains what early means in this situation: before everyone enters the room.

Because לפני means before, and when Hebrew says before followed by a whole clause, it usually uses ש־.

So:

• לפני שכולם נכנסים לחדר = before everyone enters the room

Compare:

• לפני השיעור = before the lesson
  (before + noun, no *ש־)*
• לפני שהשיעור מתחיל = before the lesson starts
  (before + clause, with *ש־)*

So ש־ is helping introduce the clause after לפני.

It is made of two parts:

• ש־ = a particle that introduces a subordinate clause
• כולם = everyone / all of them

So שכולם נכנסים לחדר means something like:

• that everyone is entering the room
• more naturally here: everyone enters the room

A useful thing to remember is that כולם literally comes from the idea of all of them, but in many contexts it is best translated as everyone.

Because its subject, כולם, is treated as plural, and the masculine plural is the normal default form for a mixed group or a group whose gender is not specified.

So:

• כולם נכנסים = everyone enters (masculine plural / mixed group / unspecified)
• כולן נכנסות = everyone enters (all female)

If the speaker wanted to make it explicitly feminine, the sentence could be:

• לפני שכולן נכנסות לחדר

The base verb is להיכנס, meaning to enter / to go in.

So:

• נכנס = enters / is entering (masculine singular)
• נכנסת = enters / is entering (feminine singular)
• נכנסים = enter / are entering (masculine plural)
• נכנסות = enter / are entering (feminine plural)

In the sentence, כולם נכנסים לחדר means everyone enters the room.

Because Hebrew commonly uses ל־ with להיכנס to mean into / to a place.

So:

• נכנסים לחדר = enter the room / go into the room

The preposition is attached directly to the noun, which is very normal in Hebrew.

A small extra note: in unpointed Hebrew, לחדר can sometimes be ambiguous in writing between to a room and to the room, but context usually makes it clear.

Yes. Hebrew word order is fairly flexible.

This sentence is natural as written:

• אני מגיעה מוקדם, כלומר לפני שכולם נכנסים לחדר.

But you could also say:

• לפני שכולם נכנסים לחדר, אני מגיעה מוקדם.

That version puts the time relationship first: Before everyone enters the room, I arrive early.

The original version sounds especially natural because it states the main idea first, then clarifies it with כלומר.

Yes, it is helpful.

The comma before כלומר shows that כלומר introduces an explanation or restatement. In writing, Hebrew often separates this kind of connector with punctuation, much like English does with that is or in other words.

So the punctuation helps the reader hear the structure:

• main statement: אני מגיעה מוקדם
• clarification: כלומר לפני שכולם נכנסים לחדר