Mi ĉiam kisas mian avinon, kiam mi vizitas ŝin dimanĉe.

Because they are direct objects.

In Esperanto, the direct object usually takes -n:

• Mi kisas mian avinon = I kiss my grandmother.
• Mi vizitas ŝin = I visit her.

So:

• avinon is the person being kissed
• ŝin is the person being visited

That -n ending is one of the most important grammar markers in Esperanto.

Because mia is acting like an adjective, and adjectives agree with the noun they describe.

Since avinon is in the accusative, mia also takes -n:

• mia avino = my grandmother
• mian avinon = my grandmother as a direct object

This agreement happens with number and accusative:

• mia avino
• mian avinon
• miaj avinoj
• miajn avinojn

Because ŝin means her, and using a pronoun avoids unnecessary repetition.

So instead of saying:

• Mi ĉiam kisas mian avinon, kiam mi vizitas mian avinon dimanĉe

Esperanto, like English, normally prefers:

• Mi ĉiam kisas mian avinon, kiam mi vizitas ŝin dimanĉe

It sounds more natural and less repetitive.

Because si is a reflexive pronoun, and it refers back to the subject of its own clause.

In the clause:

• kiam mi vizitas ŝin dimanĉe

the subject is mi.

So if you said mi vizitas sin, it would mean something like I visit myself, which is not the intended meaning.

Since the pronoun refers to my grandmother, not to the subject mi, you must use ŝin, not si.

Grammatically, kiam means when, but in a habitual sentence like this it often feels like whenever in English.

Because the sentence has:

• ĉiam = always
• present tense verbs = a habitual action

the sense is:

• I always kiss my grandmother when/whenever I visit her on Sunday/Sundays

So kiam itself is still when, but the whole sentence gives a repeated, habitual meaning.

In Esperanto, the present tense can describe a general habit, not only something happening right now.

So:

• Mi ĉiam kisas...
• mi vizitas...

does not mean the action is happening at this exact moment. It means this is what generally happens.

English does the same thing:

• I always kiss my grandmother when I visit her on Sundays.

So the present tense here expresses a repeated custom or routine.

Ĉiam means always. It shows that the action is regular or unfailing.

So the structure is:

• Mi ĉiam kisas mian avinon = I always kiss my grandmother
• kiam mi vizitas ŝin dimanĉe = when I visit her on Sunday/Sundays

Together, it means that every time that situation happens, the kissing happens too.

That is a very natural position for an adverb in Esperanto.

• Mi ĉiam kisas mian avinon...

is neutral and clear.

Esperanto word order is fairly flexible, so other placements are possible, but they may sound more marked or less natural depending on emphasis. Putting ĉiam before the verb is the most straightforward choice here.

Dimanĉe means on Sunday or on Sundays, depending on context.

The -e ending makes it an adverb. Esperanto very often uses weekday names this way:

• lunde = on Monday
• marde = on Tuesday
• dimanĉe = on Sunday

So dimanĉe is a very common and natural way to express time.

Sometimes yes, but dimanĉe is especially natural with days of the week.

Esperanto can use the accusative to show time:

• Mi venos dimanĉon = I will come on Sunday

But with weekday names, the adverbial form is extremely common:

• Mi venos dimanĉe

In this sentence, dimanĉe is the most idiomatic choice.

By itself, it can depend on context. But in this sentence, because of ĉiam and the present tense, it most naturally suggests a habitual meaning:

• on Sundays
• or whenever I visit her on Sunday

If you wanted to make a specific Sunday clearer, Esperanto often uses expressions like:

• ĉi-dimanĉe = this Sunday
• venontan dimanĉon = next Sunday

So here the meaning is probably general and habitual, not a one-time event.

Most naturally, it goes with vizitas:

• kiam mi vizitas ŝin dimanĉe = when I visit her on Sunday/Sundays

In practice, that also sets the time for the kissing, because the kissing happens during that visit. So it affects the whole situation, but grammatically it most directly modifies vizitas.

Because kiam mi vizitas ŝin dimanĉe is a subordinate clause.

Esperanto often uses commas to separate main clauses from subordinate clauses, especially when introduced by words like:

• kiam = when
• ĉar = because
• se = if
• ke = that

So the comma here helps show the structure clearly.

Because Esperanto normally states the subject explicitly in each finite clause.

So:

• Mi ĉiam kisas mian avinon, kiam mi vizitas ŝin dimanĉe

is normal.

You usually do not leave out the subject pronoun the way you can in languages like Spanish or Italian. Repeating mi keeps the sentence clear and grammatical.

• ĉ is like ch in church
• ŝ is like sh in shoe

So:

• ĉiam is pronounced roughly CHEE-am
  more exactly as two syllables: ĉi-am
• ŝin sounds roughly like sheen

These special letters are very regular in Esperanto: one letter, one sound.