الريحان بيدبل لما التربة جافة.

A natural Egyptian-style pronunciation would be:

er-rēḥān biyidbal lamma et-torba gāfa

A few notes:

• الريحان is pronounced er-rēḥān, not al-r..., because ر is a sun letter, so the l in ال assimilates.
• التربة is pronounced et-torba for the same reason, because ت is also a sun letter.
• ج in Egyptian Arabic is usually pronounced like g in go, so جافة sounds like gāfa.

Word by word:

• الريحان = the basil / basil
• بيدبل = wilts / is wilting
• لما = when
• التربة = the soil
• جافة = dry

So the structure is very close to:

The basil wilts when the soil is dry.

In Egyptian Arabic, بـ on the present tense usually marks the normal present or habitual meaning.

So:

• يدبل = wilt / to wilt / may wilt, depending on context
• بيدبل = it wilts, it does wilt, it is wilting in ordinary present usage

In this sentence, بيدبل gives the meaning of a general fact:

Basil wilts when the soil is dry.

This بـ is one of the most important features of Egyptian Arabic verbs.

Because Arabic normally leaves out the verb to be in the present tense.

So:

• التربة جافة literally = the soil dry
• natural English = the soil is dry

This is completely normal in Arabic.
If you want past or future, then a verb appears, but in the present, it is usually omitted.

Because التربة is a feminine noun, and the adjective or predicate has to agree with it.

• التربة = soil, feminine singular
• جافة = dry, feminine singular

If the noun were masculine, you would usually use the masculine form:

• جاف = dry, masculine
• جافة = dry, feminine

So جافة matches التربة.

It is functioning as the predicate in a present-tense nominal sentence.

In other words:

• التربة الجافة = the dry soil
  Here جافة is a regular adjective describing the noun directly.

But in your sentence:

• التربة جافة = the soil is dry
  Here جافة is the predicate, not just a descriptive adjective inside one noun phrase.

So the difference is:

• التربة الجافة = the dry soil
• التربة جافة = the soil is dry

Here لما means when.

So:

• لما التربة جافة = when the soil is dry

This fits a general truth or repeated situation.

It is not exactly the same as if.
If you used لو, that would sound more like a condition:

• لو التربة جافة، الريحان بيدبل = If the soil is dry, the basil wilts

Both can work in many contexts, but:

• لما = when
• لو = if

In this sentence, لما sounds natural for a general, recurring fact.

Arabic often uses the definite article more broadly than English, especially for general statements about a whole class of things.

So:

• الريحان بيدبل... literally the basil wilts...
• but the meaning is simply basil wilts...

And:

• التربة جافة = the soil is dry
• but in context it means soil in general, or the relevant soil around the plant

This is very normal. Arabic often prefers the definite form where English uses a bare noun.

It is understandable and works, but it has a bit of a mixed register feel.

What sounds Egyptian here:

• بيدبل
• لما

What sounds more formal or more Standard-Arabic-like:

• التربة
• جافة

In everyday Egyptian speech, people might choose more colloquial words, depending on context. For example, some speakers might more naturally say a version with ناشفة instead of جافة.

So the sentence is fine, but it is not the most strongly colloquial sentence possible.

The basic verb is:

• دبل = wilted
• يدبل = wilts / to wilt
• بيدبل = wilts, in the ordinary Egyptian present

A rough mini-paradigm:

• دبل = it wilted
• بيدبل = it wilts / it is wilting

So in your sentence, الريحان is treated as singular, and the verb is also singular:

• الريحان بيدبل

Yes, absolutely.

Both are natural:

• الريحان بيدبل لما التربة جافة
• لما التربة جافة، الريحان بيدبل

The second version puts more emphasis on the condition first:

When the soil is dry, the basil wilts.

So the meaning stays the same; the difference is mostly in focus and flow.

Arabic often uses a singular definite noun to talk about a plant or category in general.

So:

• الريحان can mean basil as a plant or type of plant

This is similar to how Arabic can use a singular noun to represent a whole class.
So even though English says simply basil, Arabic naturally says الريحان.

Yes, depending on the speaker, a more colloquial version might use more everyday vocabulary.

For example, many Egyptians would be more likely to use a colloquial word for dry, such as ناشفة, rather than جافة.

So you may hear something closer to:

• الريحان بيدبل لما التربة ناشفة

The exact most natural version can vary by speaker and context, but the main grammar stays the same:

• subject
• present verb with بـ
• لما
• a present-tense nominal sentence without is

So your original sentence is very useful for learning the structure, even if some words are slightly more formal.