אחת הסיבות שהיא באה מוקדם היא שהיא לא רוצה לאחר בכוונה, כי זה מנומס יותר.

Because סיבות is a feminine plural noun. The singular is סיבה, which is feminine, so the word for one has to be the feminine form אחת.

• masculine: אחד
• feminine: אחת

So:

• אחת הסיבות = one of the reasons
• אחד הדברים = one of the things

אחת הסיבות means one of the reasons.

סיבה אחת means one reason.

That difference is important:

• סיבה אחת = just one reason
• אחת הסיבות = one out of a known set of reasons

So the pattern אחת ה... often means one of the ...:

• אחת הבעיות = one of the problems
• אחד הילדים = one of the children

It is really two parts written together:

• ש־ = that
• היא = she

So שהיא literally means that she.

In this sentence it appears twice:

• הסיבות שהיא באה מוקדם = the reasons that she comes/is coming early
• היא שהיא לא רוצה... = is that she does not want...

Hebrew very often attaches ש־ directly to the next word.

That היא is part of the structure X היא Y, which works like X is Y.

Here the sentence is built like this:

• אחת הסיבות ... היא ...
• One of the reasons ... is ...

So:

• הסיבה היא ש... = the reason is that...
• הבעיה היא ש... = the problem is that...

In English, we use is. In Hebrew, in this kind of sentence, you often see a pronoun like הוא / היא instead of a present-tense form of to be.

Because the subject is היא = she.

Hebrew verbs agree with the subject in gender and number. So with a feminine singular subject, you get a feminine singular form.

Compare:

• הוא בא = he comes / came
• היא באה = she comes / came

So באה matches she.

Good question: באה can be either:

• present feminine singular: comes / is coming
• past feminine singular: came

They look the same in writing.

You decide from context. In this sentence, present is the natural reading because the rest of the sentence is also present:

• היא לא רוצה = she does not want
• זה מנומס יותר = it is more polite

So the sentence most naturally reads as something like she comes/is coming early, not she came early.

If it were clearly past, you would usually expect the rest to be past too, for example לא רצתה instead of לא רוצה.

Here it functions like an adverb: early.

Literally, מוקדם is an adjective meaning early or early/advanced depending on context, but Hebrew often uses adjective forms adverbially.

So:

• היא באה מוקדם = she comes early / arrives early

You do not need a separate adverb form the way English has early, quickly, etc.

Another similar example:

• הוא מדבר מהר = he speaks fast

Here לאחר means to be late or to arrive late.

It comes from the same root as words connected with lateness or delay. In this sentence:

• לא רוצה לאחר = does not want to be late

A learner may notice that in other contexts the same root can also relate to delay. That is normal. Hebrew roots often cover a family of related meanings.

Because the Hebrew infinitive usually already includes ל־, which often corresponds to English to.

So:

• לאחר = to be late
• לבוא = to come
• ללכת = to go

That means לא רוצה לאחר is literally structured like:

• does not want to-be-late

There is no extra separate word needed.

בכוונה means on purpose or intentionally.

Here it modifies לאחר:

• לאחר בכוונה = to be late on purpose

So the idea is not just lateness, but deliberate lateness.

Its position here is natural and clear because it sits right after the infinitive it belongs to.

Because זה refers to the whole action or behavior, not to the woman herself.

• זה מנומס יותר = that is more polite
• היא מנומסת יותר = she is more polite

Those are different meanings.

In this sentence, the speaker is judging the behavior of not being late on purpose. So Hebrew uses זה to mean something like that / this behavior / that idea.

This is very common in Hebrew:

• לא לצעוק זה חשוב = not shouting is important
• לבוא בזמן זה מנומס = coming on time is polite

יותר means more.

So:

• מנומס = polite
• מנומס יותר = more polite

Hebrew does not need a word for than unless you state the second side of the comparison explicitly.

Here the comparison is understood from context: being early is more polite than being late on purpose.

If you do want to say than, Hebrew usually uses מ־:

• יותר מנומס לאחר = more polite than to be late
• הוא גבוה ממני = he is taller than me

So in this sentence, than is just implied rather than spoken.

Yes. כי is a very common and natural way to say because in everyday Hebrew.

So:

• כי זה מנומס יותר = because it is more polite

Hebrew also has other ways to say because, such as מפני ש־ or בגלל ש־, but כי is short, common, and completely normal here.