Kariri msamiati huu leo, la sivyo utasahau maneno haya kesho.

Kukariri is the infinitive, meaning to memorize.
When Swahili gives a direct command to one person, it usually uses the bare verb stem:

• kukariri = to memorize
• kariri = memorize!

So the ku- disappears in the imperative.

In Swahili commands, the subject you is usually understood and does not need to be stated. So kariri already means memorize! addressed to one person.

This is very normal in Swahili, just like English can say Come here! without explicitly saying you.

They are related, but not identical:

• msamiati = vocabulary, meaning a set of words or lexical items as a group
• maneno = words, the individual words themselves

So the sentence first talks about this vocabulary as a set, then warns that you will forget these words individually.

In Swahili, demonstratives such as this and these normally come after the noun, not before it as in English.

So:

• msamiati huu = this vocabulary
• maneno haya = these words

That word order is standard Swahili.

Because Swahili demonstratives must agree with the noun class of the noun they describe.

• msamiati takes class 3 singular agreement here, so it uses huu
• maneno is a class 6 plural noun, so it uses haya

This is a very important feature of Swahili grammar: words like demonstratives, adjectives, and verbs often change form to match the noun class.

La sivyo is a fixed expression meaning otherwise, if not, or if you don’t do that.

In this sentence, it connects the command with the consequence:

• Kariri msamiati huu leo = Memorize this vocabulary today
• la sivyo utasahau maneno haya kesho = otherwise you will forget these words tomorrow

It is best learned as a set phrase.

Utasahau can be broken down like this:

• u- = you (singular subject marker)
• -ta- = future tense marker
• -sahau = forget

So utasahau literally means you will forget.

Here, msamiati is being treated as a singular collective noun, meaning vocabulary. Even though vocabulary contains many words, the noun itself is singular in this sentence, which is why it takes huu.

That is similar to English vocabulary, which also often refers to many words as one whole set.

To more than one person, you would normally use the plural imperative and plural subject marker:

Karirini msamiati huu leo, la sivyo mtasahau maneno haya kesho.

Breakdown:

• karirini = memorize! (to more than one person)
• mta- = you plural will

To make it softer or more polite, you could also add tafadhali:

Tafadhali, kariri msamiati huu leo...
or
Tafadhali, karirini msamiati huu leo...