هذا ضمن الخطة.

Because Arabic often uses nominal sentences in the present tense, with no written or spoken verb equivalent to English is / are.

So:

• هذا = this
• ضمن الخطة = within the plan / part of the plan

Together, the sentence means This is within the plan or This is part of the plan, even though Arabic does not say is explicitly here.

If you wanted a past-tense version, Arabic would usually add a verb, for example:

• كان هذا ضمن الخطة = This was part of the plan

Here ضمن means within, inside, or more naturally in many contexts, as part of.

In this sentence, it often has the sense of:

• within the plan
• included in the plan
• part of the plan

So ضمن is very useful when you want to say that something falls inside the limits of a plan, category, system, or arrangement.

Examples:

• ضمن القانون = within the law / under the law
• ضمن الاتفاق = within the agreement / covered by the agreement

Yes, in sentences like this, learners can treat ضمن as a preposition-like word meaning within or among.

It is followed by a noun in the genitive relationship, so:

• ضمن الخطة
• not ضمن الخطةُ
• not ضمن الخطةَ

In fully vowelled Arabic, you would see:

• ضِمْنَ الخِطَّةِ

That final -i on الخطةِ shows the genitive case.

Because الخطة means the plan, while خطة means a plan.

So:

• ضمن الخطة = within the plan
• ضمن خطة = within a plan

If the meaning shown to the learner is something like This is part of the plan, then الخطة makes sense because it refers to a specific plan already known from the context.

هذا is the demonstrative pronoun meaning this for a masculine singular noun or referent.

In this sentence, هذا is the topic of the sentence: this.

The structure is:

• هذا = the subject/topic
• ضمن الخطة = the predicate/comment about it

So the sentence is literally structured like:

• This — within the plan

which in natural English becomes:

• This is within the plan
• This is part of the plan

هذا is the masculine singular form of this.

If the thing referred to is feminine singular, you would normally use:

• هذه = this (feminine)

So you might say:

• هذه ضمن الخطة = This is part of the plan (for a feminine referent)

Examples:

• هذا المشروع ضمن الخطة = This project is part of the plan
• هذه الفكرة ضمن الخطة = This idea is part of the plan

So the demonstrative should usually match the gender of the noun it refers to.

It is in the genitive case, because it comes after ضمن.

In fully vowelled MSA, the sentence would be:

• هٰذا ضِمْنَ الخِطَّةِ

Breakdown:

• هٰذا = demonstrative
• ضِمْنَ = preposition-like word
• الخِطَّةِ = the plan in the genitive

In normal unvowelled writing, learners usually just see:

• هذا ضمن الخطة

So even though the case ending is usually not written, the grammar is still there.

A careful MSA pronunciation is approximately:

• hādhā ḍimna al-khiṭṭah
• or in fully case-marked reading: hādhā ḍimna al-khiṭṭati

A few pronunciation notes:

• هٰذا = hādhā
• ض in ضمن is a heavy/emphatic d-like sound
• خ in الخطة is like the ch in Scottish loch or German Bach
• ط is an emphatic t
• الخ in الخطة keeps the l of al-, because خ is a moon letter, not a sun letter

So you pronounce it as:

• al-khiṭṭa not
• akh-khiṭṭa

Yes, you might hear or use في الخطة in some contexts, but ضمن and في are not always exactly the same.

A rough difference:

• في الخطة = in the plan
• ضمن الخطة = within the plan / as part of the plan

ضمن often sounds a bit more like something is included within the scope of the plan.
في is more general and simply means in.

In many situations both can work, but ضمن is often the better choice when the idea is included as part of something.

This is a nominal sentence (جملة اسمية).

Its basic structure is:

• هذا = mubtadaʾ (the subject/topic)
• ضمن الخطة = the khabar (the predicate)

The predicate here is not a single noun or adjective. It is a prepositional phrase telling you where or in what scope this belongs.

So grammatically, Arabic is saying:

• This — within the plan

That is a very common Arabic pattern.

Usually ضمن الخطة means within the plan or part of the plan, not exactly according to the plan.

If you want according to the plan, Arabic more often uses expressions like:

• حسب الخطة
• وفقًا للخطة

So the meanings are close, but not identical:

• ضمن الخطة = included in the plan
• حسب الخطة / وفقًا للخطة = according to the plan

That distinction is useful for sounding more precise in MSA.