الجيب في البنطال الأبيض. السترة ملونة.

In Modern Standard Arabic, the verb to be is usually not written or said in the present tense.

So:

• الجيب في البنطال الأبيض
• السترة ملونة

literally look like:

• the pocket in the white pants
• the jacket colorful/colored

But in natural English, you translate them with is.

This is called a nominal sentence in Arabic.
If you wanted was, will be, and so on, Arabic would use a verb such as كان.

Because in Arabic, adjectives normally follow the noun they describe.

So:

• البنطال الأبيض = the white pants
• not الأبيض البنطال

This is the normal order in Arabic:

• noun + adjective

The same pattern appears all the time:

• الكتاب الجديد = the new book
• السيارة الكبيرة = the big car

Because when an adjective directly describes a noun in Arabic, the adjective must agree with the noun. One part of that agreement is definiteness.

So if the noun is definite with ال, the adjective must also be definite with ال:

• البنطال الأبيض = the white pants

If the noun were indefinite, the adjective would also be indefinite:

• بنطال أبيض = white pants / a white pair of pants

This matching is a very important Arabic pattern.

These two mean different things grammatically.

• السترة ملونة = The jacket is colorful/colored
• السترة الملونة = the colorful/colored jacket

In السترة ملونة, ملونة is the predicate of the sentence, not an adjective inside a noun phrase. In this structure, Arabic usually leaves the predicate indefinite, so there is no ال on ملونة.

So:

• السترة ملونة = a full sentence
• السترة الملونة = just a noun phrase

This is one of the most common things learners notice in Arabic.

Because السترة is a feminine singular noun, and the adjective/predicate agrees with it in gender.

So:

• سترة is feminine
• therefore ملونة is feminine

The masculine form would be:

• ملون

For example:

• القميص ملون = The shirt is colorful/colored
• السترة ملونة = The jacket is colorful/colored

So the ة here marks the feminine form.

In Arabic, البنطال is treated as a singular masculine noun, even though in English pants/trousers is plural in form.

So Arabic says:

• البنطال الأبيض
  literally: the white pants/trousers

And the adjective is singular masculine:

• الأبيض

This is normal. English and Arabic do not always classify nouns the same way.

Yes. في means in, and it is a preposition. In Arabic, a preposition makes the following noun genitive.

So in a fully vocalized version, you would have something like:

• الجيبُ
• في البنطالِ الأبيضِ

After في, both البنطال and its adjective الأبيض would take genitive case endings.

In normal unvoweled writing, you usually do not see those endings, but they are still understood grammatically.

No. The pronunciation of ال changes depending on the next letter.

• In السترة, the next letter is س, which is a sun letter.
  The ل of ال is not pronounced, and the س is doubled in pronunciation.

• In البنطال, the next letter is ب, which is a moon letter.
  The ل of ال is pronounced normally.

So this is a pronunciation rule, not a spelling change: the word is still written with ال either way.

Often, yes. Arabic commonly uses ال when the speaker is referring to a specific, identifiable thing, especially in simple descriptions like this.

So:

• الجيب = the pocket
• السترة = the jacket

In a context like describing a picture or a known object, Arabic often prefers the definite form.

If you made them indefinite, the meaning would become less specific:

• جيب = a pocket
• سترة = a jacket

So the definiteness here helps show that we are talking about particular items.

ملونة is an adjective meaning something like colored or colorful, depending on context.

It comes from the root related to color and is built from the verb لوّن (to color).

Its basic forms are:

• ملون = masculine singular
• ملونة = feminine singular

So in your sentence:

• السترة ملونة

the feminine form is used because السترة is feminine.