البرنامج الجديد مختلف عن البرنامج القديم، لكنه ليس صعبا.

It is a nominal sentence in Arabic.

• البرنامج الجديد = the subject/topic, literally the new program
• مختلف عن البرنامج القديم = the comment/predicate, literally different from the old program

Arabic does not need a present-tense word for is here, so مختلف by itself can mean is different in context.

Because الجديد is an adjective describing البرنامج, and Arabic adjectives must agree with the noun in definiteness.

So:

• البرنامج الجديد = the new program
• برنامج جديد = a new program

If the noun is definite, the adjective must also be definite.

Because here مختلف is not part of the noun phrase the new program. It is the predicate of the sentence, meaning is different.

So:

• البرنامج الجديد = the new program
• مختلف = is different

In Arabic, predicates like this are usually indefinite, so مختلف appears without الـ.

Because the normal Arabic pattern is مختلف عن = different from.

English says:

• different from

Arabic says:

• مختلف عن

So عن is simply the preposition that goes with مختلف in standard Arabic.

Yes, Arabic can sometimes say عن القديم if the meaning is already clear, and that would mean from the old one / the old program.

But repeating the noun:

• عن البرنامج القديم

makes the sentence clearer and more explicit, especially in formal or careful writing. Arabic often repeats nouns where English might prefer to avoid repetition.

لكنه means but it or but however it in this sentence.

It is made of:

• لكن = but / however
• ـه = it / he

Here ـه refers back to البرنامج.

In fully vowelled Arabic, it is written لكنَّهُ.
A longer form, ولكنه, is also possible and is common in formal writing.

Because ليس is the normal way to negate a nominal sentence like this one.

So:

• هو صعب = it is difficult
• هو ليس صعبًا = it is not difficult

In Arabic, لا is usually used with verbs or in other specific types of negation, but for a sentence like it is not difficult, ليس is the standard choice.

In fully vowelled Arabic, it is صعبًا.

That is because صعبًا is the predicate of ليس, and the predicate of ليس is normally in the accusative case.

So the full form is:

• ليس صعبًا

In ordinary unvocalized writing, case endings are often not shown, but when they are shown, this word takes fatḥatayn: صعبًا.

برنامج is a masculine singular noun, so the words describing it are also masculine singular:

• الجديد = new
• القديم = old
• مختلف = different
• صعبًا = difficult

If the noun were feminine, these forms would change. For example:

• الرسالة الجديدة ليست صعبةً
• The new message is not difficult

So Arabic adjectives and similar descriptive words must match the noun in gender and number.

A fully vowelled version is:

البرنامجُ الجديدُ مختلفٌ عن البرنامجِ القديمِ، لكنَّهُ ليس صعبًا.

The endings show the grammar:

• البرنامجُ الجديدُ → nominative subject
• مختلفٌ → nominative predicate
• عن البرنامجِ القديمِ → genitive after the preposition عن
• صعبًا → accusative predicate after ليس

This is a good example of how case endings work in formal Modern Standard Arabic.