Yeye anaandika katika shajara yake jioni.

Yes. In Swahili, anaandika already includes the subject marker a-, which means he/she.

So the sentence could simply be:

Anaandika katika shajara yake jioni.

Adding yeye gives extra emphasis, a bit like saying:

• He/She is the one writing...
• As for him/her, he/she writes...

In many everyday sentences, Swahili leaves out the independent pronoun unless it is needed for emphasis or contrast.

Anaandika can be broken down like this:

• a- = he/she
• -na- = present tense / ongoing action
• andika = write

So anaandika means he/she writes or he/she is writing, depending on context.

This is very common in Swahili: a verb often includes the subject and tense inside the word itself.

It can mean either one, depending on context.

In Swahili, the -na- tense often covers both:

• he/she writes
• he/she is writing

So this sentence could describe:

• a habitual action: He/She writes in his/her diary in the evening
• an action happening now in the evening: He/She is writing in his/her diary this evening

The wider context usually makes the meaning clear.

Yake means his/her/its for certain noun classes.

Here, the noun is shajara (diary/journal), and it belongs to a noun class that takes the possessive form ya-. Then -ke gives the meaning his/her.

So:

• shajara yake = his/her diary

A useful thing to notice is that Swahili does not distinguish between his and her here. The same form, yake, can mean either one.

Because the possessive must agree with the noun class of shajara, not with the gender of the owner.

English speakers often expect the possessive to match the person, but in Swahili it matches the noun being possessed.

So:

• shajara takes ya-
• ya- + -ke becomes yake

That is why you get:

• shajara yake = his/her diary

By contrast, wake is used with noun classes that take wa-, usually referring to people, for example:

• mtoto wake = his/her child

Katika means in, inside, or sometimes within.

In this sentence:

• katika shajara yake = in his/her diary

It is a more formal or careful way to express location. In everyday speech, many speakers might also use other forms such as kwenye, depending on context and style.

So katika is correct and natural, but it can sound a little more formal than some alternatives.

Yes, in many contexts you could.

Both can refer to in/on the diary, depending on how the speaker views the location. In real usage, kwenye is often very common in everyday Swahili.

Very roughly:

• katika = in/inside, often a bit more formal
• kwenye = common everyday locative, often translated as in, on, or at depending on context

So:

• Anaandika katika shajara yake = correct
• Anaandika kwenye shajara yake = also very natural in many situations

Shajara usually means diary, journal, or sometimes a written record of events.

Depending on context, it can suggest:

• a personal diary
• a journal
• a logbook
• a written record

So the English translation may vary slightly, but diary is a very common and reasonable choice here.

Jioni means evening or in the evening.

Swahili often does not need a separate preposition like English does. A time word can function on its own:

• asubuhi = morning / in the morning
• mchana = daytime / in the daytime
• jioni = evening / in the evening
• usiku = night / at night

So jioni by itself already gives the time meaning naturally.

That is a very natural place for a time expression in Swahili.

The sentence structure here is roughly:

• Yeye = subject
• anaandika = verb
• katika shajara yake = place/location
• jioni = time

Putting jioni at the end sounds normal and clear. Swahili does allow some flexibility, though, especially for emphasis.

For example, you may also hear:

• Jioni, yeye anaandika katika shajara yake.

That version emphasizes in the evening more strongly.

It can change somewhat, but the original order is very natural.

The basic sentence:

• Yeye anaandika katika shajara yake jioni.

Possible variations include:

• Anaandika katika shajara yake jioni.
• Jioni anaandika katika shajara yake.
• Katika shajara yake anaandika jioni.
  This is possible, but it sounds more marked or stylistically special.

Swahili word order is often more flexible than English, especially when the speaker wants to emphasize a certain part of the sentence.

You do not know from this sentence alone.

Swahili commonly uses forms that do not show gender:

• yeye = he or she
• a- in the verb = he/she
• yake = his/her

To know whether the person is male or female, you need context from the conversation or text.

This is one major difference from English: Swahili usually does not mark gender in pronouns and possessives the way English does.

Not if you mean writing in the diary.

In English, write a diary is not the normal expression; we usually say write in a diary or write in one’s diary. Swahili works similarly here.

So:

• anaandika katika shajara yake = he/she writes in his/her diary

If you removed katika, the meaning would change or become unnatural, because shajara is the place/medium where the writing happens, not the direct object in the usual sense.

Yes, it is natural and correct.

A few notes:

• Yeye adds emphasis, so without special emphasis many speakers might simply say Anaandika...
• katika is correct and slightly more formal than some everyday alternatives
• jioni at the end is perfectly natural

So the sentence is good Swahili, and a learner can safely use it.