هذه مجموعة.

Because مجموعة is a grammatically feminine noun, and the demonstrative has to match it.

• هذا = this for masculine singular nouns
• هذه = this for feminine singular nouns

Since مجموعة ends in ة and is feminine, Arabic uses هذه.

Yes. In Arabic, the present-tense verb to be is usually not stated.

So Arabic can simply say:

• هذه مجموعة = This is a group

This is a normal nominal sentence in Arabic. English needs is, but Arabic usually does not in the present tense.

In basic grammar terms:

• هذه is the demonstrative: this
• مجموعة is the predicate: it tells you what this is

In traditional Arabic grammar, you can think of them as:

• هذه = مبتدأ (the starting element / topic)
• مجموعة = خبر (the information given about it)

So the structure is essentially:

• this
  • a group

The noun مجموعة is feminine because it has the ending ة (taa marbuuTa), which very often marks feminine nouns in Arabic.

That does not always mean the thing itself is biologically female. It just means the word is treated as feminine in grammar.

So even though group is not a female person, مجموعة is still grammatically feminine.

In careful Modern Standard Arabic, هذه is pronounced approximately:

• hādhihi

A few pronunciation notes:

• هـ = h
• ا after h gives a long ā
• ذ sounds like th in this
• the final ه is part of the written form, and the word is commonly taught as hādhihi

So an English-friendly approximation is:

• HAA-thi-hi

But remember that the th is the voiced sound from this, not the unvoiced one from thin.

A common careful pronunciation is:

• majmūʿa

If you want the fully inflected form, it can be:

• majmūʿatun

Some key parts:

• مَجْ = maj
• مُو = mū
• ع = the letter ʿayn, a sound English does not really have
• final ة is usually heard as -a / -ah when pausing

So if you are a beginner, maj-MOO-ah is a useful rough approximation, but try to learn the ع sound over time.

The final ة is called taa marbuuTa.

Its pronunciation depends on the situation:

• At the end of a pause, it is usually pronounced like -a or -ah
  
  • مجموعة → majmūʿa
• When fully pronounced with case endings, it can sound like t
  • مجموعةٌ → majmūʿatun

So the letter is written as ة, but you do not always hear a clear t at the end.

Because here مجموعة means a group, not the group.

So:

• هذه مجموعة = This is a group
• هذه المجموعة = this group

That second example, هذه المجموعة, is usually a noun phrase meaning this group, not the full sentence This is the group.

So the absence of ال helps show that مجموعة is indefinite here.

In fully vocalized, formal grammar, yes, the word would be:

• هذه مجموعةٌ

That final ٌ marks an indefinite nominative ending, because مجموعة is the predicate of the sentence.

However:

• Most everyday Arabic writing does not show short vowels or tanwīn
• At the end of a sentence, speakers often pause and do not pronounce the final -un

So in normal writing, هذه مجموعة is completely standard.

Usually, for this simple sentence, you would just say:

• هذه مجموعة

Adding هي is generally unnecessary here.

Arabic often uses no present-tense to be, so the shorter version is the normal one.

If هي is added in other contexts, it usually gives extra emphasis or helps clarify structure, but for a basic sentence like this, learners should treat هذه مجموعة as the standard pattern.

Yes. مجموعة comes from the root:

• ج م ع

This root has meanings connected with gathering or collecting.

That is useful because many Arabic words are built from roots, so recognizing the root can help you learn related words more easily. For example, words from ج م ع often have meanings related to collecting, joining, or gathering.

So learning مجموعة is also a small step toward recognizing a whole word family.