Somo hili ni magumu.

Not in standard Swahili. The usual form would be Somo hili ni gumu.

Why? Because somo is singular, so the adjective should also be singular: gumu.
The form magumu is plural, so it matches masomo:

• Somo hili ni gumu. = This lesson is difficult.
• Masomo haya ni magumu. = These lessons are difficult.

Ni is the copula, the word that links the subject to what is being said about it. In English, this is often is/are.

So in Somo hili ni gumu, ni works like is:

• Somo hili = this lesson
• ni = is
• gumu = difficult / hard

Unlike English, ni does not change here for singular or plural.

Swahili uses different forms of this depending on the noun class of the noun.

Somo belongs to noun class 5 in the singular, and the class-5 form of this is hili.

So:

• somo hili = this lesson

If the noun were plural, somo becomes masomo, and this/these changes too:

• masomo haya = these lessons

In Swahili, demonstratives like this and that usually come after the noun.

So Swahili says:

• somo hili = lesson this

not the English-style order this lesson.

This is normal Swahili word order. You will see the same pattern in many phrases:

• mtu huyu = this person
• kitabu hiki = this book
• masomo haya = these lessons

Because gumu is the singular form that matches somo, while magumu is the plural form.

The adjective stem is -gumu. Its form changes to agree with the noun class:

• class 5 singular: gumu
• class 6 plural: magumu

So:

• somo gumu = a difficult lesson
• masomo magumu = difficult lessons

In a full sentence:

• Somo hili ni gumu.
• Masomo haya ni magumu.

Somo is in the ji/ma noun class pair:

• singular: somo
• plural: masomo

This matters because words connected to the noun must agree with its class. That includes words like this and many adjectives.

So the agreement looks like this:

• somo hili = this lesson
• somo ... gumu = lesson ... difficult
• masomo haya = these lessons
• masomo ... magumu = lessons ... difficult

Swahili grammar depends a lot on this agreement system.

You would say:

Masomo haya ni magumu.

Changes:

• somo → masomo
• hili → haya
• gumu → magumu

So everything changes to match the plural noun.

Yes, if you do not need to say this specifically.

• Somo ni gumu. = The lesson is difficult. / A lesson is difficult.
• Somo hili ni gumu. = This lesson is difficult.

So hili adds the idea of this.

Yes, but it means something slightly different.

• somo gumu = a difficult lesson / difficult lesson
• Somo hili ni gumu = this lesson is difficult

In other words:

• somo gumu is a noun phrase
• Somo hili ni gumu is a full sentence

So if you want to describe the lesson inside a phrase, somo gumu is fine. If you want to make a complete statement, use ni:

• Hili ni somo gumu. = This is a difficult lesson.
• Somo hili ni gumu. = This lesson is difficult.