مبلغ القرض ونسبة الفايدة بيغيروا القسط الشهري.

Because مبلغ القرض is an iḍāfa construction, often called a construct phrase.

• مبلغ = amount
• القرض = the loan

Together, مبلغ القرض means the amount of the loan.

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun shows whether the whole phrase is definite

So مبلغ القرض is already definite because القرض is definite.

The same thing happens in نسبة الفايدة.

نسبة الفايدة is another iḍāfa:

• نسبة = rate / percentage
• الفايدة = the interest

So the whole phrase means the interest rate.

As for the spelling, فايدة is the common Egyptian form of MSA فائدة. In Egyptian Arabic, people usually pronounce it something like fāyda, so the spelling فايدة reflects the spoken form more closely.

In finance:

• فايدة = interest

In other contexts, it can also mean:

• benefit
• usefulness

بيغيروا breaks down like this:

• بـ = present / habitual marker
• يغير = change
• وا = they

So بيغيروا means they change or they are changing, depending on context.

Here it is from the verb غيّر, which is a transitive verb, so it means to change something.

That is different from يتغير, which means to change / become different in the sense of happening to the thing itself.

So here:

• بيغيروا القسط الشهري = they change the monthly payment

Because the subject is made of two things joined by و:

• مبلغ القرض
• نسبة الفايدة

Together, they function as a plural subject, so Egyptian Arabic normally uses a plural verb:

• مبلغ القرض ونسبة الفايدة بيغيروا...

If there were only one subject, the verb would be singular:

• مبلغ القرض بيغير القسط الشهري = The loan amount changes the monthly payment
• نسبة الفايدة بتغير القسط الشهري = The interest rate changes the monthly payment

Notice that نسبة is feminine, so with that alone you would usually get بتغير.

In Egyptian Arabic, بـ on the imperfect verb usually marks the ordinary present:

• habitual actions
• general facts
• repeated actions
• ongoing present, depending on context

So بيغيروا here fits a general statement, something like:

• These factors change / affect the monthly payment

Without بـ, the verb would usually not sound like the normal everyday present in Egyptian Arabic.

Because قسط is a masculine noun.

Adjectives in Arabic agree with the noun they describe in:

• gender
• number
• definiteness

So:

• القسط = the installment / the payment
• الشهري = monthly

Since القسط is masculine singular definite, the adjective must also be masculine singular definite:

• القسط الشهري

If the noun were feminine, you would expect الشهرية instead.

Yes, this is very normal in Egyptian Arabic.

The sentence starts with the subject:

• مبلغ القرض ونسبة الفايدة = the loan amount and the interest rate

Then comes the verb:

• بيغيروا

Then the object:

• القسط الشهري

This subject–verb–object order is very common in spoken Egyptian Arabic, especially in clear explanatory sentences like this one.

It also helps explain why the verb shows full plural agreement here. When the subject comes first, full agreement is very natural.

Literally, بيغيروا means change.

But in natural English, this sentence is often better understood as affect or determine:

• The loan amount and the interest rate affect the monthly payment

So the Arabic verb is literally stronger, but the natural English equivalent may sometimes be a little smoother.

This is common in Arabic: a verb like غيّر may be used where English would choose affect.

This is about moon letters and sun letters.

In القرض:

• ق is a moon letter
• so the ل in الـ is pronounced

So it sounds roughly like:

• el-ʔarḍ or il-ʔarḍ

In الشهري:

• ش is a sun letter
• so the ل sound assimilates to ش

So it sounds roughly like:

• esh-shahri or ish-shahri

That is why learners often hear:

• القرض with a clear l
• الشهري without a clear l

A rough pronunciation would be:

mablagh el-ʔarḍ wi nesbet el-fāyda byeghayyaru el-ʔest esh-shahri

A few useful notes:

• و is often pronounced wi
• ق is often a glottal stop ʔ in Egyptian speech
• الشهري is pronounced with assimilation: esh-shahri
• الفايدة is commonly pronounced el-fāyda

So if you say it in a Cairene-style pronunciation, it may sound close to:

mablagh el-ʔarḍ wi nesbet el-fāyda byeghayyaru el-ʔest esh-shahri