Puerum verborum iratorum paenitet, et sorori magnas gratias agit quod ei ignoscit.

Because paenitet is usually used as an impersonal verb in Latin.

With paenitet, Latin does not treat the person who feels regret as the grammatical subject the way English does. Instead:

• the person who feels regret goes in the accusative
• the thing regretted goes in the genitive
• paenitet itself stays in the 3rd person singular

So:

• puerum paenitet = the boy feels regret / the boy regrets

Even though the boy is the one experiencing the feeling, Latin puts him in the accusative here.

Again, this is because paenitet is being used impersonally.

It does not mean something like the boy regrets in a fully personal grammatical way. It is more like:

• it causes regret to the boy

That is why the verb remains 3rd person singular: paenitet.

If there were several people regretting something, Latin could still use the same impersonal verb, with the people in the accusative:

• pueros paenitet = the boys regret

So the number of the regretful person does not control the verb in the usual way.

Because with paenitet, the thing regretted is commonly put in the genitive.

So:

• puerum verborum iratorum paenitet
  = the boy regrets angry words

Literally, it is something like:

• the boy feels regret of angry words

That sounds strange in English, but it is normal Latin grammar with this verb.

Iratorum modifies verborum.

Both are genitive plural, so they agree:

• verborum = of words
• iratorum = angry

Together:

• verborum iratorum = of angry words

So iratorum is not separate from verborum; it is simply the adjective describing the words.

Because the expression gratias agere takes the person thanked in the dative.

So:

• sorori magnas gratias agit = he gives great thanks to his sister
• more naturally: he thanks his sister very much

This is a common Latin pattern:

• alicui gratias agere = to thank someone

So sorori is dative because she is the person receiving the thanks.

It is an idiomatic Latin expression meaning he gives great thanks or simply he thanks very much.

Word by word:

• magnas = great
• gratias = thanks
• agit = does / drives / gives in this idiom

But you should learn the whole phrase together:

• gratias agere = to give thanks / thank
• magnas gratias agere = to give great thanks, to thank warmly

So this is not a literal English-style phrase; it is a standard Latin way to say thank you or to thank someone.

Here quod means because.

It is not a relative pronoun here. It is introducing a clause that gives the reason:

• quod ei ignoscit = because ... forgives ...

So in this sentence, quod is a causal conjunction.

Latin can use both quod and quia for because, and here quod is perfectly normal.

Because ignoscere takes the person forgiven in the dative.

So:

• ei ignoscit = he/she forgives him/her

This is different from English, where we say forgive someone with a direct object. Latin instead thinks more like:

• to grant forgiveness to someone

So the person forgiven is dative:

• ei = to him / to her

The subject is understood from context, not stated explicitly.

Grammatically, ignoscit just means he forgives or she forgives, because Latin often leaves subject pronouns unstated when they are not needed.

In this sentence, the most natural understanding is:

• the boy regrets his angry words, and thanks his sister greatly because she forgives him

Why is that the most natural reading?

• sorori has just been mentioned
• ei can mean to him
• it makes good sense that he thanks his sister because she forgives him

Could ignoscit theoretically mean he forgives her? Grammatically, yes, if ei referred back to sorori. But in context that is much less likely.

So the implied subject of ignoscit is most naturally the sister.

Latin often leaves possessive words unstated when they are obvious from context.

English frequently says:

• his sister
• his angry words

Latin may simply say:

• sorori
• verborum iratorum

and let the reader understand whose sister and whose words are meant.

So Latin does not always use suus or another possessive adjective when the relationship is already clear.

Latin word order is more flexible than English word order, because the endings show the grammatical function of each word.

So the sentence can place words for emphasis or style:

• Puerum verborum iratorum paenitet
• et sorori magnas gratias agit
• quod ei ignoscit

Some things to notice:

• puerum comes first, which gives prominence to the boy
• paenitet comes later, after the phrase explaining what he regrets
• ei ignoscit places ei before the verb, which can help highlight the person being forgiven

So the order is not random, but it is also not as rigid as English. The endings, not the position alone, tell you how the sentence works.