أنا هنا منذ وقت طويل.

In Arabic, the verb to be is usually not written in the present tense.

So:

• أنا هنا = I am here
• literally: I here

This is completely normal in Modern Standard Arabic.
If you want past or future, Arabic does use a verb:

• كنت هنا = I was here
• سأكون هنا = I will be here

So in this sentence, the present-tense am is simply understood.

You usually do need أنا here, because the sentence has no verb to show who the subject is.

• أنا هنا منذ وقت طويل = I have been here for a long time
• هو هنا منذ وقت طويل = He has been here for a long time

Without أنا, the listener may not know who here since a long time refers to.

So أنا is important because it identifies the subject.

أنا هنا is a very normal and straightforward order in Arabic.

It is a nominal sentence:

• أنا = the subject
• هنا = the predicate / location

So the basic idea is:

• I — here

Arabic often puts the subject first in simple present-tense statements like this.

You may also see different word orders in other contexts for emphasis, but أنا هنا is the standard, natural order.

منذ is used for time and often means either:

• since
• or for

The exact English translation depends on context.

In this sentence:

• أنا هنا منذ وقت طويل

the natural English meaning is:

• I have been here for a long time

But literally, منذ has the idea of time extending from some point up to now.

Compare:

• منذ الأمس = since yesterday
• منذ ساعة = for an hour / since an hour ago
• منذ وقت طويل = for a long time

So منذ is the key word that gives the sense of duration up to the present.

Arabic and English express this idea differently.

English often uses:

• I have been here for a long time

Arabic usually expresses the same meaning with:

• a present-state sentence: أنا هنا
• plus a time expression with منذ

So Arabic does not need a separate equivalent of have been here.
The meaning of continuing up to now comes from the whole structure:

• أنا هنا = I am here
• منذ وقت طويل = since/for a long time

Together, they naturally mean:

• I have been here for a long time

Literally, وقت طويل means:

• a long time

Word by word:

• وقت = time
• طويل = long

So the phrase is an adjective + noun combination meaning long time.

Arabic also has other common ways to say this idea, such as:

• منذ زمن طويل
• منذ مدة طويلة

All of them can mean for a long time, though the exact style may differ a bit.

Because طويل describes وقت, and وقت is a masculine noun.

In Arabic, adjectives agree with the nouns they describe in gender, number, and definiteness.

So:

• وقت طويل = masculine noun + masculine adjective
• مدة طويلة = feminine noun + feminine adjective

That is why we say:

• وقت طويل not
• وقت طويلة

It is indefinite:

• وقت طويل = a long time

There is no ال on either word, so both are indefinite.

If you said:

• الوقت الطويل

that would mean something like the long time, which is not the normal expression here.

After منذ, Arabic naturally uses the indefinite phrase:

• منذ وقت طويل = for a long time

A simple pronunciation is:

• anā hunā mundhu waqtin ṭawīlin

A few helpful notes:

• أنا = anā
• هنا = hunā
• منذ = mundhu
• وقت = waqt
• طويل = ṭawīl

In normal reading without full case endings, many learners will just say:

• anā hunā mundhu waqt ṭawīl

That is very common in practice.

Yes, in fully vocalized formal Arabic, you could analyze it as:

• أنا هنا منذُ وقتٍ طويلٍ

Here:

• منذُ ends with -u
• وقتٍ is genitive after منذ
• طويلٍ agrees with وقتٍ, so it is also genitive

However, in most everyday reading and speaking of MSA, these endings are often not pronounced fully, especially at pause.

So learners usually first encounter it simply as:

• أنا هنا منذ وقت طويل

That may be a literal step toward the meaning, but it is not natural English.

The natural English translation is:

• I have been here for a long time

Arabic uses a present-style structure, but English normally uses the present perfect for this idea.

So when translating, it is better not to mirror the Arabic word-for-word.

Yes, you could.

• أنا موجود هنا منذ وقت طويل

means something like:

• I am present / I am here and have been here for a long time

But موجود is not necessary in the original sentence.
أنا هنا already means I am here.

Adding موجود can sometimes sound a little more explicit or emphatic, but the shorter version is perfectly normal.

Yes. Besides منذ وقت طويل, you may also see:

• منذ زمن طويل
• منذ مدة طويلة

All are common and understandable.

Roughly:

• وقت = time
• زمن = time / era / period
• مدة = duration / period

For a learner, it is enough to know that all three can express for a long time, though the exact nuance may vary slightly.

In this sentence, yes, that is the most natural order:

• أنا هنا منذ وقت طويل

First you state the situation:

• I am here

Then you add the duration:

• for a long time

Arabic can sometimes move parts of the sentence around for emphasis, but this order is the clearest and most neutral for learners.

It is a nominal sentence in Arabic.

The core is:

• أنا هنا = a present-tense nominal sentence

Then Arabic adds a prepositional time phrase:

• منذ وقت طويل

So the structure is basically:

• subject: أنا
• predicate/location: هنا
• time expression: منذ وقت طويل

This is a very common Arabic pattern for expressing states and locations in the present.