لا أستعمل هذه الكاميرا كثيرا، لكن شاشة هاتفي صغيرة ولذلك أحتاج إلى كاميرا جديدة.

Because لا is the normal way to negate a present-tense verb in MSA.

• أستعمل = I use
• لا أستعمل = I do not use

ليس is usually used to negate nominal sentences, not a present-tense verb like أستعمل. So here لا is the natural choice.

The prefix أ- marks first person singular in the present tense, so it means I.

• أستعمل = I use
• تستعمل = you use or she uses
• يستعمل = he uses
• نستعمل = we use

So you can think of أستعمل as the verb form already containing the subject I.

Because كاميرا is treated as a feminine singular noun in Arabic, so it takes the feminine demonstrative هذه.

• هذا = this for masculine singular nouns
• هذه = this for feminine singular nouns

So:

• هذه الكاميرا = this camera

That is a very common question. In standard Arabic, when a demonstrative directly modifies a noun, the noun is usually definite, so it normally takes الـ.

So Arabic says:

• هذه الكاميرا
• literally something like this the-camera

But that is just normal Arabic grammar. It is the standard way to say this camera.

كثيرًا means a lot, much, or sometimes often, depending on context.

In this sentence:

• لا أستعمل هذه الكاميرا كثيرا
• = I do not use this camera much / very often

It comes after the verb phrase and works adverbially. In a negative sentence like this, English often translates it as much or very much.

They are very close in meaning. Both can mean I use.

• أستعمل = I use
• أستخدم = I use

In many contexts, either works. Depending on speaker and region, one may sound a bit more common than the other, but both are standard and understandable in MSA.

Yes. Both لكن and ولكن can mean but.

• لكن is very common and natural
• ولكن is also correct and can sound a bit more formal or fuller

So this sentence could use لكن naturally, as it does here.

In Arabic, the verb to be is usually not expressed in the present tense in sentences like this.

So:

• شاشة هاتفي صغيرة
• literally: the screen of my phone small
• natural English: The screen of my phone is small

This is called a nominal sentence.

If you wanted past or future, Arabic would usually add a form of كان:

• كانت شاشة هاتفي صغيرة = The screen of my phone was small

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

• شاشة = screen
• هاتفي = my phone
• شاشة هاتفي = the screen of my phone / my phone’s screen

In an iḍāfa:

1. the first noun usually does not take الـ
2. the second noun determines the definiteness of the whole phrase

Since هاتفي means my phone, the whole phrase is definite:

• شاشة هاتفي = the screen of my phone

It is the noun هاتف plus the possessive suffix -ي meaning my.

• هاتف = phone
• هاتفي = my phone

So Arabic often expresses possession by attaching a suffix directly to the noun.

Other examples:

• كتابي = my book
• بيتي = my house

It is feminine because it refers to شاشة, and شاشة is a feminine noun.

• صغير = small masculine
• صغيرة = small feminine

It does not have الـ because here it is the predicate of the sentence, not an adjective inside the noun phrase.

So:

• شاشة هاتفي صغيرة
• = The screen of my phone is small

If صغيرة were directly modifying a definite noun inside one noun phrase, the pattern would be different. But here it is the statement being made about the subject.

ولذلك is made up of three parts:

• و = and
• لِ = for / because of
• ذلك = that

Together, ولذلك means:

• and therefore
• and for that reason
• so

It connects the second idea to the first: the phone screen is small, therefore a new camera is needed.

Because the verb احتاج in MSA normally takes the preposition إلى.

So the standard pattern is:

• أحتاج إلى شيء
• = I need something

Therefore:

• أحتاج إلى كاميرا جديدة
• = I need a new camera

In some spoken varieties, people may use different patterns, but in MSA إلى is the expected choice.

Because in Arabic, adjectives usually come after the noun they describe.

So:

• كاميرا جديدة = a new camera

Also, the adjective must agree with the noun. Since كاميرا is feminine singular, the adjective is also feminine singular:

• جديد = new masculine
• جديدة = new feminine

So كاميرا جديدة is the correct form.