ať: Zavolej jim, ať to neposílají.Call them not to send it.
nechť: Řekni mu, nechť mi to pošle.Tell him to send it to me.
stihnout: Nestihl jsem to poslat.I didn't manage to send it.
whom: I want to send a telegram. – To whom?Chci poslat telegram. – Komu?