Shame on me! Если бы вы хоть изредка открывали учебник, то знали бы...

[VIadimir]

Ну, бывает с каждым... В общем, мой позорный вариант:

Had you ever opened your books, you would know...
Had you opened your textbooks? once in a while, you'd know...
If you had opened your books sometimes, you would've known...

Какой вариант? Может, что-то своё подскажете, плиз?
И ещё: как сказать "С такими помощниками он и напакостить нормально не сможет"... Мой ???вар.:
1. With such a miserable help?, he'd hardly be able to play his nasty/mean tricks. ?
2. With an? assistance as pitiable/pitiful/piteous as his (получается, что он сам - помощник! Как переделать?), he wouldn't even...
3. His toadies' aid is a joke! It's a fat chance he'd ever be able to...
4. Considering those schmucks surrounding him, it's a slim chance he'd...
5. Given all those wimps that make up his retinue, it's a zero chance...

[d.]

If you opened ... you'd know..

[segu]

With helpers like these...
help - неисчисляемое
THERE is a zero chance...

If you took the trouble to open ... you'd know

[VIadimir]

Ой, спасибо! Как Вы меня спасли от позора!:) Точно, there's...
Насчёт trouble - классно!
А перевод "напакостить" нормальный? Чё-то у меня сомнения.

[Aly19]

Может, еще play dirty tricks
или do mischief

[VIadimir]

Спасибо, Aly19! mischief возьму.