الروح قوية.

Because Arabic often uses a nominal sentence in the present tense, with no copula (no separate word for is/am/are).

So:

• الروح قوية = The spirit is strong

In past or future contexts, Arabic does use words equivalent to was or will be, but in the simple present, they are normally left out.

Because الروح is a grammatically feminine noun, and the adjective must agree with it.

So:

• الروح = feminine noun
• قوية = feminine form of strong

This is standard adjective agreement in Arabic:

• masculine: قوي
• feminine: قوية

Even if the noun does not refer to a female person, it can still be grammatically feminine.

You often have to learn the gender of each noun, because grammatical gender does not always match natural gender.

In this case, روح is treated as feminine in Modern Standard Arabic, so its adjective appears in the feminine form:

• روح قوية = a strong spirit

A helpful clue is that many nouns referring to abstract ideas are feminine, but this is not a rule you can always rely on.

This is a very common question.

In Arabic, when an adjective comes after a noun inside a noun phrase, it usually matches the noun in definiteness:

• الروح القوية = the strong spirit
• روح قوية = a strong spirit

But in الروح قوية, the word قوية is not an attributive adjective inside the noun phrase. It is the predicate of the sentence.

So the structure is:

• الروح = the subject
• قوية = the predicate, meaning strong

That is why قوية does not take ال here.

They mean different things structurally:

• الروح قوية = The spirit is strong
  This is a full sentence.

• الروح القوية = the strong spirit
  This is a noun phrase, not a full sentence by itself.

So the absence or presence of ال on قوية changes the grammar.

In الروح, the ل of ال is not pronounced fully, because ر is a sun letter.

So instead of al-rūḥ, it is pronounced more like:

• ar-rūḥ

The r sound is doubled.

If fully vowelled, it is:

• الرُّوحُ
• pronounced roughly ar-rūḥu in full grammatical pronunciation

In normal pausal pronunciation, you will often hear simply:

• ar-rūḥ

The ending ـة (called tāʾ marbūṭa) usually marks the feminine form of a noun or adjective.

Here:

• قوي = strong (masculine)
• قوية = strong (feminine)

Since الروح is feminine, the adjective takes this feminine ending.

In pause, ـة is usually pronounced like -a:

• qawiyya

But in connected grammatical reading with case endings, it can sound like a t before the case vowel.

In fully vowelled Classical or formal grammatical pronunciation, it is:

• الرُّوحُ قَوِيَّةٌ

That is:

• الرُّوحُ = subject, so it takes ـُ / -u
• قَوِيَّةٌ = predicate, so it also takes ـٌ / -un

In everyday reading and speech, these endings are usually not pronounced, so learners commonly see and say:

• الروح قوية

It is a nominal sentence (جملة اسمية), because it starts with a noun.

Its two main parts are:

• المبتدأ = the subject/topic → الروح
• الخبر = the predicate/comment → قوية

So the sentence literally works like:

• The spirit — strong

which in natural English becomes:

• The spirit is strong

In normal neutral style, الروح قوية is the standard order.

Arabic can move parts of a sentence for emphasis in some contexts, but for a beginner, the important basic pattern is:

• noun + predicate
• الروح قوية

So this is the best form to learn first.