Mi legis ĉiun kondiĉon pri resendo.

Legis is the past tense of legi = to read.

• mi legas = I read / I am reading
• mi legis = I read / I have read
• mi legos = I will read

So Mi legis... tells you the reading happened in the past.

Because they are both part of the direct object.

In Esperanto, the direct object usually takes -n.
Here, the thing that was read is ĉiun kondiĉon.

Also, words that go together must agree with each other. So:

• ĉiu kondiĉo = every condition
• ĉiun kondiĉon = every condition, as a direct object

Both the determiner ĉiu and the noun kondiĉo take -n here.

Because ĉiu means each / every and is normally used with a singular noun.

So:

• ĉiu kondiĉo = each condition / every condition
• ĉiuj kondiĉoj = all conditions

That means:

• Mi legis ĉiun kondiĉon = I read every single condition, one by one
• Mi legis ĉiujn kondiĉojn = I read all the conditions

In many situations, both are possible, but ĉiun kondiĉon emphasizes the individual items.

Ĉiu means each, every, or sometimes everyone, depending on context.

With a singular noun, it means every / each:

• ĉiu tago = every day
• ĉiu persono = each person
• ĉiu kondiĉo = every condition

Used by itself, it can mean each one or everyone:

• Ĉiu venis. = Everyone came.

So in this sentence, ĉiun kondiĉon means every condition.

Pri usually means about, concerning, or regarding.

So:

• kondiĉo pri resendo = a condition about return/shipping back
• demando pri la problemo = a question about the problem
• libro pri historio = a book about history

In this sentence, pri resendo tells you what kind of conditions they were.

Resendo is built from:

• sendi = to send
• re- = again / back
• -o = noun ending

So resendo literally means something like a sending back or return shipment.

This is a very normal Esperanto way to build words. The learner should notice:

• sendi = to send
• resendi = to send back
• resendo = sending back / return

Depending on context, English may translate it as return, return shipment, or sending back.

Because after the preposition pri, you normally do not use the accusative -n.

Prepositions in Esperanto generally govern their own phrase, so:

• pri resendo = about return
• pri la kondiĉoj = about the conditions
• kun amiko = with a friend

The -n ending is not used there unless there is some special reason unrelated to pri itself. In this sentence, resendo is simply the object of the preposition pri, so it stays resendo.

Yes, you could.

Esperanto word order is fairly flexible because the endings show the grammar clearly. Since ĉiun kondiĉon has -n, it is clearly the object no matter where it appears.

So all of these are grammatically possible:

• Mi legis ĉiun kondiĉon pri resendo.
• Ĉiun kondiĉon pri resendo mi legis.

The first is the most neutral.
The second gives more emphasis to ĉiun kondiĉon pri resendo.

Yes. That would also be natural, but the meaning shifts a little.

• Mi legis ĉiun kondiĉon pri resendo. = I read every condition; emphasis on each individual condition.
• Mi legis la kondiĉojn pri resendo. = I read the return conditions / the conditions about returns; more like referring to the set as a whole.

So the version with ĉiun stresses completeness item by item, while la kondiĉojn is more like the conditions as a group.

It is a noun.

You can tell because it has the noun ending -o in its basic form:

• kondiĉo = condition

In the sentence it appears as kondiĉon, with the extra -n because it is the direct object.

So:

• base form: kondiĉo
• object form: kondiĉon

Not exactly.

• resendo comes from resendi = to send back
• redono comes from redoni = to give back / return

Sometimes both can relate to the idea of returning something, but the nuance is different:

• resendo focuses on sending back
• redono focuses on giving back

For store policies, resendo can sound especially appropriate when the item is physically shipped back.