المشكلة بسيطة.

Because Arabic often uses a nominal sentence in the present tense, with no overt verb for to be.

So:

• المشكلة بسيطة
• literally: the problem simple
• natural English: The problem is simple

This is completely normal in Modern Standard Arabic.
If you wanted the past tense, you would usually add كان:

• كانت المشكلة بسيطة
• The problem was simple

This is a basic nominal sentence made of:

• المشكلة = the subject
• بسيطة = the predicate

In traditional Arabic grammar:

• المشكلة is the مبتدأ (mubtadaʾ)
• بسيطة is the خبر (khabar)

So the structure is:

• [subject] + [description/information about the subject]

Here, the sentence tells us something about the problem: it is simple.

Because المشكلة is a feminine singular noun, and the predicate adjective must agree with it in gender and number.

So:

• المشكلة بسيطة = The problem is simple
• not المشكلة بسيط

The noun مشكلة is feminine because it ends in ة (taa marbuuTa), which is a very common sign of feminine nouns.

Compare:

• المشكلة بسيطة = the problem is simple
• الأمر بسيط = the matter is simple

Here الأمر is masculine, so the adjective is بسيط.

Because in this sentence, المشكلة is the definite subject: the problem.

But بسيطة is the predicate, and predicates are often indefinite in Arabic.

So:

• المشكلة = the problem
• بسيطة = simple

This is the normal pattern for a sentence like The problem is simple.

If you made both words definite, it would usually change the meaning or sound unnatural in this context.

It is originally an adjective, but in this sentence it is functioning as the predicate.

That means it is still a descriptive word, but it is not directly modifying a noun inside a noun phrase. Instead, it is making a complete statement about the subject.

Compare these two:

• مشكلة بسيطة = a simple problem
  Here بسيطة is a normal adjective modifying مشكلة inside one phrase.

• المشكلة بسيطة = The problem is simple
  Here بسيطة is the predicate of the whole sentence.

So the same adjective can be used in both ways.

A careful pronunciation is:

• al-mushkilatu basiiTatun

In normal pause, it is usually pronounced more like:

• al-mushkila basiiTa

A few notes:

• مشكلة = mushkila
• بسيطة = basiiTa
• ط is an emphatic t
• the long ii sound in بسيطة is held a bit longer than a short vowel

In everyday reading, case endings are often not pronounced fully unless you are speaking very formally or reading aloud carefully.

With full case endings, it is:

• المشكلةُ بسيطةٌ

Why?

• المشكلةُ takes -u because it is the mubtadaʾ (subject of a nominal sentence), so it is in the nominative
• بسيطةٌ also takes -un because the predicate is usually nominative too

In normal unvowelled writing, these endings are not shown, so you usually just see:

• المشكلة بسيطة

The final ة is called taa marbuuTa.

In these words, it marks them as feminine singular:

• مشكلة
• بسيطة

In pause, it is usually pronounced like -a:

• mushkila
• basiiTa

But in certain grammatical contexts, especially when endings are fully pronounced or in connected speech, it can sound like t:

• المشكلةُ = al-mushkilatu
• بسيطةٌ = basiiTatun

So the written ة often sounds like a at the end of a paused word, but it is historically and grammatically a kind of t.

The normal, neutral order in a nominal sentence is:

• subject + predicate

So:

• المشكلة بسيطة

You can move the predicate first in some contexts for style, emphasis, or special sentence conditions, but the basic learner-friendly pattern is the one you see here.

So for now, think of this as the standard structure:

• The problem
  • is simple

In المشكلة, the ال is pronounced clearly as al-, because م is a moon letter.

So you say:

• al-mushkila

not something like am-mushkila

This matters because with sun letters, the l sound of ال assimilates, but with moon letters it stays pronounced.

Since م is a moon letter, the pronunciation stays al-.

Yes. This is one of the most important basic patterns in Arabic.

You can build many present-tense descriptive sentences this way:

• البيت كبير = The house is big
• السماء صافية = The sky is clear
• الولد ذكي = The boy is smart
• السيارة جديدة = The car is new

So المشكلة بسيطة is a great example of a very common Arabic sentence type:

• definite noun + predicate
• with no present-tense verb for is

You would say:

• مشكلة بسيطة

This is a noun + adjective phrase, meaning:

• a simple problem

Notice the difference:

• مشكلة بسيطة = a simple problem
• المشكلة بسيطة = the problem is simple

So the presence of ال on المشكلة helps show that the second one is a full sentence, not just a noun phrase.

In this case, yes: مشكلة is grammatically feminine, and the ة is a strong clue.

More generally:

• many nouns ending in ة are feminine
• but grammatical gender in Arabic must ultimately be learned word by word

For مشكلة, you should simply memorize:

• مشكلة = feminine
• therefore: مشكلة بسيطة
• and: المشكلة بسيطة

So the agreement in this sentence is exactly what you should expect.