"Tengo que salir antes de las siete."
Translation:I have to leave before seven.
I don't know, exactly. It never accepts numerals for me, either. I have reported it several times, but nothing seems to have been done about it.
It's a set phrase that you just have to accept :)
"To have to do something" = "Tener que hacer algo".
"Tener que" has a colloquial Spanish meaning that can translate to "I must."