"Il veut un pull alors sa mère lui en tricote un."

Translation: He wants a sweater, so his mother is knitting him one.

[AndreaVout]

Knitting him one ... so why "en" and "un" ?

[Rae781]

It's essentially saying "one of them". Using just "un" would be "she knits one" but in French you need to specify one of what

[DrSBenson]

He wants a sweater, so his mother is knitting one FOR him.

[JCw4bx]

"He wants a sweater so his mother is knitting him one of them" was marked wrong.
I think DL should accept it. If the boy wanted an ice-cream, an anglophone would say "He wants an ice-cream so his mother is buying him one of them'' . To say "...one of it" sounds awkward.

[UlfPersson3]

'he wants a sweater so his mother-in-law knits him one' is rejected. True it does not emphasise that the knitting is still going on and is not concluded . The English sentence is not ungrammatical and has a meaning, how would it be translated into English? (One could always check with Google translate but I have little confidence in it). One could also say ´... so his mother proceeds to knit him one' which Duo no doubt would reject.

[Lisaskier]

Why the need for both "EN" and "UN" for "one" ?

[Jamaleen]

Because 'en' can also mean 'one of them', which is the sense here.

[Bekir978479]

Is it possible to say "...sa mere tricote le lui.

[GayMarchant]

I used pullover, same as sweater, but not accepted.

[WayneEEdge]

He wants a sweater so his mother knits him one....wrong really?
Duo put ...is knitting him one.