Ο πεθερός της λέει ότι όταν ήταν παιδί, ήταν φρόνιμος στην τάξη αλλά ανυπόμονος στο σπίτι.

Here της means her. It is a weak genitive pronoun used to show possession, so Ο πεθερός της literally means the father-in-law of her, which in natural English is her father-in-law.

A useful thing to remember is that the same form της can also mean to her in other sentences, but here it is clearly possessive because it follows the noun πεθερός.

Πεθερός means father-in-law. It refers to a relationship by marriage.

It does not mean stepfather. The Greek word for stepfather is πατριός.

Λέει is present tense because the reporting happens now: her father-in-law says...

What he says, however, is about his childhood, so that part is in the past: όταν ήταν παιδί, ήταν φρόνιμος... This is very similar to English: He says that when he was a child...

Ότι introduces a content clause, like English that.

So λέει ότι... means he says that... In everyday Greek, you may also hear πως instead of ότι, and in casual speech the that-word can sometimes be omitted altogether.

After είμαι or ήταν, Greek often leaves out the article when talking about someone's role, identity, or category.

So ήταν παιδί means he was a child. If you said ήταν το παιδί, that would sound much more specific, like he was the child or he was the kid in a particular context.

Because παιδί is the noun child, and that noun is grammatically neuter in Greek no matter whether the child is a boy or a girl.

But φρόνιμος and ανυπόμονος describe the actual person being talked about, namely ο πεθερός, who is masculine. So the adjectives agree with the person, not with the noun παιδί.

If the sentence were about a woman, you would get feminine forms, for example φρόνιμη and ανυπόμονη.

Greek often leaves subject pronouns out because they are understood from context. This is very normal.

Here the subject is already clear from Ο πεθερός της, so Greek does not need to repeat αυτός. English usually needs he, but Greek often does not.

Ήταν is the imperfect form of είμαι, meaning was. It is the normal form for describing a state or condition in the past.

It appears twice because there are two separate ideas:

• όταν ήταν παιδί = when he was a child
• ήταν φρόνιμος... αλλά ανυπόμονος... = he was well-behaved... but impatient...

So each clause needs its own verb.

They are contractions of σε plus the definite article:

• σε + την = στην
• σε + το = στο

In this sentence:

• στην τάξη = in class / in the classroom
• στο σπίτι = at home / in the house

This contraction is extremely common in Greek.

In this context, φρόνιμος means something like well-behaved, good, or sensible, especially for a child.

So ήταν φρόνιμος στην τάξη suggests that he behaved properly at school. It is not mainly about being intellectually clever; it is more about behavior and self-control.

Ανυπόμονος means impatient.

Here it suggests that, although he behaved well in class, he was less patient or more restless at home. So the sentence contrasts his behavior in two different settings.

Greek word order is fairly flexible, and this sentence could be rearranged without changing the basic meaning.

For example, you could also say Ο πεθερός της λέει πως ήταν φρόνιμος στην τάξη αλλά ανυπόμονος στο σπίτι όταν ήταν παιδί, though the original version is clearer because όταν ήταν παιδί comes before the description it modifies.

So the original order is natural and easy to follow, but it is not the only possible one.