En nia familio ni ofte pruntas librojn unu al la alia kaj redonas ilin post unu semajno.

Yes. En normally means in, and here En nia familio means In our family.

It is a very natural way in Esperanto to set the context first:

• En nia familio = in our family / in our household / among our family members

So it does not necessarily mean physically inside a family; it means within the family group.

Possessive words like mia, via, nia, lia, ŝia, ilia usually do not need la before them.

So:

• nia familio = our family

Using la here would usually sound unnecessary or wrong in normal usage.

Because familio is not the direct object of the verb.

In this sentence:

• ni is the subject = we
• librojn is the direct object = books

But en nia familio is just a prepositional phrase. After a preposition like en, nouns normally do not take -n unless there is a special reason involving direction. Here there is no movement into something, just location/context.

So:

• en nia familio = correct
• not en nia familion

Ofte means often. Esperanto word order is fairly flexible, but adverbs like ofte are commonly placed near the verb they modify.

So:

• ni ofte pruntas = we often lend/borrow

This is very natural. You could also see other word orders in Esperanto, but this placement is straightforward and common.

This is a very common question, because Esperanto handles this idea a little differently from English.

Prunti can be used in contexts involving lending/borrowing, but very often you clarify the direction with the rest of the sentence.

Here:

• pruntas librojn unu al la alia = lend books to one another

Because of unu al la alia (to each other), the meaning is clearly reciprocal: family members lend books to each other.

In many learning materials, you may also meet:

• pruntedoni = lend
• pruntepreni = borrow

Those are extra-clear forms built from:

• doni = give
• preni = take

So this sentence is understood as mutual lending.

Because librojn is the direct object of pruntas.

• libroj = books
• librojn = books as the thing being acted on

The family members are doing the action of lending/borrowing, and books are what is involved in that action, so Esperanto marks them with the accusative -n.

Unu al la alia means to one another or to each other.

Literally it is:

• unu = one
• al = to
• la alia = the other

So literally: one to the other.

Even though English thinks of this as plural (each other), Esperanto often uses this singular-style expression to show reciprocity between members of a group.

So:

• ni pruntas librojn unu al la alia = we lend books to each other

This is a standard Esperanto pattern.

Because alia here is being used as a noun-like expression meaning the other one.

So:

• alia = other
• la alia = the other one

That makes unu al la alia a set phrase:

• one to the other
• meaning to each other

Without la, it would sound less standard here.

Redonas means gives back or returns.

It is built from:

• donas = gives
• re- = again / back

So:

• redonas = gives back, returns

In context:

• redonas ilin = returns them

This is a very common and useful Esperanto word.

Because ilin is the direct object form of ili.

• ili = they
• ilin = them

Here ilin refers back to librojn:

• redonas ilin = return them

Since the books are the thing being returned, Esperanto uses the accusative -n.

Because the subject is still the same: ni.

In Esperanto, just like in English, if two verbs share the same subject and are joined by kaj (and), you do not need to repeat the subject.

So:

• ni ofte pruntas ... kaj redonas ... means
• we often lend ... and return ...

Repeating ni would be possible for emphasis, but it is not necessary.

Because post is a preposition, and after a preposition Esperanto normally uses the basic noun form, not the accusative.

So:

• post unu semajno = after one week

By contrast, unu semajnon without a preposition can mean something like for one week as a duration of time.

Compare:

• post unu semajno = after one week
• dum unu semajno = for one week / during one week
• mi restis unu semajnon = I stayed for a week

Yes. In natural English, post unu semajno is often translated as:

• after a week or
• after one week

Both are fine, depending on context. Esperanto uses unu both for the number one and for English a/an in some expressions of time and quantity like this.

Yes, it is very natural.

The structure is clear and typical:

• En nia familio = context
• ni ofte pruntas librojn unu al la alia = main action
• kaj redonas ilin post unu semajno = second action with the same subject

It is a good example of several common Esperanto patterns:

• possessives like nia
• accusative objects like librojn and ilin
• reciprocity with unu al la alia
• a prefixed verb like redonas
• a time phrase with post