Ni baada ya mawimbi kutulia ndipo watoto waliingia majini.

This pattern is used for emphasis. In this sentence, Ni baada ya mawimbi kutulia ndipo watoto waliingia majini means something like:

• It was after the waves had calmed down that the children went into the water.
• More naturally: Only after the waves had calmed down did the children go into the water.

So ni ... ndipo ... highlights the time expression baada ya mawimbi kutulia and makes it the key point of the sentence.

A useful way to think of it is:

• ni = marks focus/emphasis
• ndipo = that is when / then

This is a very common Swahili way of emphasizing when something happened.

After baada ya (after), Swahili often uses the infinitive form of the verb, which begins with ku-.

So:

• baada ya kutulia = after calming down / after becoming calm

In your sentence, the infinitive phrase has a subject inside it:

• baada ya mawimbi kutulia = after the waves calmed down

This structure is very common:

• baada ya kula = after eating
• baada ya kufika = after arriving
• baada ya watoto kulala = after the children slept / after the children went to sleep

So kutulia is not strange here at all — it is exactly the form Swahili normally uses after baada ya.

Kutulia means to become calm, settle down, become still, quiet down.

With mawimbi (waves), it means the waves became calmer or less rough.

So in this sentence, mawimbi kutulia means:

• the waves calming down
• the waves becoming calm

Depending on context, kutulia can also be used for people, noise, emotions, and situations:

• Mtoto ametulia = The child is calm.
• Bahari imetulia = The sea is calm.
• Tulia! = Calm down! / Be quiet!

Mawimbi is the plural of wimbi (wave).

It belongs to the JI/MA noun class:

• singular: wimbi
• plural: mawimbi

This class often has:

• singular with no obvious prefix or with ji-
• plural with ma-

Examples:

• jicho / macho = eye / eyes
• tunda / matunda = fruit / fruits
• wimbi / mawimbi = wave / waves

Because mawimbi is plural, other words in the sentence may relate to that plural idea, though here kutulia is an infinitive, so it does not show agreement.

Waliingia can be broken into:

• wa- = subject marker for they (people, noun class 2)
• -li- = past tense marker
• -ingia = verb root enter / go in

So:

• waliingia = they entered / they went in

Since watoto (children) is a plural human noun, the subject marker is wa-.

Compare:

• mtoto aliingia = the child entered
• watoto waliingia = the children entered

Because watoto is a plural human noun, and plural human nouns usually take the class 2 agreement marker wa-.

Here is the singular/plural pair:

• mtoto = child
• watoto = children

And the verb agrees with that:

• mtoto aliingia = the child entered
• watoto waliingia = the children entered

So wa- in waliingia is there because the subject is watoto.

This is one of the most important agreement patterns in Swahili:

• singular person: a-
• plural people: wa-

Majini comes from maji (water) plus the locative ending -ni.

So:

• maji = water
• majini = in the water / into the water / at the water

In this sentence, because the verb is -ingia (enter / go into), majini is best understood as:

• into the water

So watoto waliingia majini means the children went into the water.

The exact English translation depends on the verb and context:

• with a location verb, majini may mean in the water
• with -ingia, it naturally means into the water

Swahili often expresses location and movement using the locative ending -ni instead of a separate word like English in, into, or at.

So:

• nyumba = house
• nyumbani = at home / in the house / to the house, depending on context

Likewise:

• maji = water
• majini = in/into the water

Then the verb helps clarify the meaning:

• kuwa majini = to be in the water
• kuingia majini = to go into the water

So the idea of into comes from the combination of:

1. the locative form majini
2. the motion verb kuingia

Ndipo often means something like:

• then
• that is when
• that is where, depending on context

In this sentence, it means that is when.

So the structure is roughly:

• Ni baada ya mawimbi kutulia ndipo ...
• It was after the waves had calmed down that ...

You will often see ndipo in sentences that emphasize time:

• Ni jana ndipo nilimwona = It was yesterday that I saw him/her.
• Ni hapo ndipo walikaa = That is where they stayed.

So in your sentence, ndipo helps introduce the main event after the emphasized time phrase.

Yes. A simpler version would be:

• Baada ya mawimbi kutulia, watoto waliingia majini.

This also means:

• After the waves had calmed down, the children went into the water.

The difference is emphasis:

• Baada ya mawimbi kutulia, watoto waliingia majini.
  = neutral statement

• Ni baada ya mawimbi kutulia ndipo watoto waliingia majini.
  = stronger emphasis: it was only after that that the children entered the water

So the longer version is not more “correct” — it is just more emphatic.

More or less, yes. If you look at it word by word:

• baada ya = after
• mawimbi = waves
• kutulia = to calm down / to become calm

So literally it is something like:

• after the waves to become calm

But in natural English, we say:

• after the waves had calmed down
• after the waves calmed down

This is a good example of how Swahili often uses the infinitive in places where English uses a full clause.

Because after baada ya, Swahili normally prefers the infinitive-style construction rather than a fully conjugated verb.

So the natural pattern is:

• baada ya mawimbi kutulia

not usually:

• baada ya mawimbi yalitulia

The second version sounds wrong or at least very unnatural in standard Swahili in this structure.

So a good rule for learners is:

• after baada ya, expect ku- forms such as kufika, kula, kuondoka, kutulia

Examples:

• baada ya mvua kuisha = after the rain stopped
• baada ya wageni kufika = after the guests arrived
• baada ya watoto kula = after the children ate

It can mean both, depending on context.

The verb kuingia means:

• to enter
• to go in
• to go into

So watoto waliingia majini can be understood as:

• the children entered the water
• the children went into the water

In English, went into the water is usually the most natural translation here.

Swahili often uses -ingia in places where English might choose either enter or go into.

This is a good question because maji looks plural and behaves a bit differently from English water.

In everyday learning terms, it is best to treat maji as a fixed noun meaning water. It often belongs to the ma- class in form, but its meaning is usually uncountable in English.

What matters for this sentence is mainly the locative form:

• maji = water
• majini = in/into the water

So even though English water is uncountable, Swahili uses the noun maji in its own grammatical way.

A helpful breakdown is:

• Ni baada ya mawimbi kutulia = It was after the waves had calmed down
• ndipo watoto waliingia majini = that the children went into the water

So the full structure is:

1. an emphasized time phrase
2. ndipo
3. the main clause

You can think of it as:

• [Focused time expression] + ndipo + [main event]

This pattern is very useful in Swahili when you want to stress when something happened.