המילה הזאת ביחיד, אבל במשפט הבא היא באה ברבים.

Because מילה is a feminine noun in Hebrew.

• this with a masculine singular noun: הזה
• this with a feminine singular noun: הזאת (or sometimes זו in other styles)

So:

• הספר הזה = this book
• המילה הזאת = this word

The demonstrative must agree with the noun in gender and number.

Hebrew often uses pronouns according to the grammatical gender of the noun, not natural gender.

Since מילה is feminine, the sentence refers back to it with היא = she/it.

In English, we would say it, but in Hebrew:

• המילה ... היא ...

That is completely normal. Hebrew does not have a separate everyday pronoun that works exactly like English it.

Because it agrees with היא, which refers to המילה, a feminine singular noun.

The verb בא means came / comes, and in Hebrew it changes for gender and number:

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

So in this sentence:

• היא באה ברבים = it appears in the plural

Even though English would not mark gender on the verb, Hebrew does.

ביחיד literally means in the singular.

It is made of:

• ב־ = in
• יחיד = singular

So המילה הזאת ביחיד means this word is in the singular.

This is a common Hebrew way to talk about grammatical number:

• ביחיד = in the singular
• ברבים = in the plural

ברבים means in the plural.

It is formed from:

• ב־ = in
• רבים = plural / many

So:

• היא באה ברבים = it comes/appears in the plural

This is the standard grammatical term opposite ביחיד.

In the present tense, Hebrew often leaves out the verb to be.

So where English says:

• This word is in the singular

Hebrew often says literally:

• This word in-singular

That is normal Hebrew sentence structure.

If you wanted to say it in the past or future, then a form of to be would be expressed differently:

• המילה הזאת הייתה ביחיד = this word was in the singular

But in the present, no separate is is needed.

הבא means next or following.

So:

• במשפט הבא = in the next sentence

It comes from the root ב־ו־א / בוא, related to coming, so the idea is something like the coming sentence.

A few examples:

• בשיעור הבא = in the next lesson
• בעמוד הבא = on the next page
• ביום הבא = on the following day

Because משפט is a masculine noun.

The adjective הבא must agree with the noun it describes:

• masculine singular: הבא
• feminine singular: הבאה

So:

• משפט הבא would need masculine agreement
• במשפט הבא = in the next sentence

Compare:

• במילה הבאה = in the next word
  because מילה is feminine

Yes, literally בא/באה means comes or came, but Hebrew often uses it idiomatically to mean something like:

• appears
• occurs
• is found
• is used

So היא באה ברבים does not mean the word is physically coming somewhere. It means:

• the word appears in plural form
• the word is used in the plural in the next sentence

This is a very natural Hebrew expression.

No. In modern Hebrew, you may also see:

• המילה הזאת
• המילה הזו

Both mean this word.

Some learners first meet הזה / הזאת, while זה / זו and הזו also appear in real usage. For this sentence, המילה הזאת is completely standard and natural.

The sentence is:

• המילה הזאת ביחיד, אבל במשפט הבא היא באה ברבים.

Its structure is roughly:

• This word
  • in the singular
• but
• in the next sentence
• it
• appears in the plural

A more literal breakdown:

• המילה הזאת = this word
• ביחיד = in the singular
• אבל = but
• במשפט הבא = in the next sentence
• היא = it
• באה = comes/appears
• ברבים = in the plural

Hebrew word order is flexible, but this order is very natural.

If the noun being discussed were masculine, the agreeing words would change.

For example, with a masculine noun like שם (name):

• השם הזה ביחיד, אבל במשפט הבא הוא בא ברבים.

Notice the changes:

• הזה instead of הזאת
• הוא instead of היא
• בא instead of באה

So this sentence is a good example of how Hebrew agreement works across demonstratives, pronouns, and verbs.