What it means
てはいけないから means “because someone must not do something” or “because something is not allowed.” It combines a verb’s te-form with てはいけない and から, making the prohibition the reason for another statement. The familiar form uses てはいけないから, while the polite counterpart uses てはいけませんから.
Usage note
Despite the possible translation “because it cannot be done,” this form usually expresses prohibition or lack of permission, not physical inability. For a reason that is stated more neutrally or tentatively, other forms such as ので may be more appropriate.
How to use it
この部屋では電話をかけてはいけませんから、廊下でかけましょう。
Because we must not make phone calls in this room, let’s make phone calls in the hallway.
この部屋では電話をかけてはいけないから、廊下でかけよう。
Because we must not make phone calls in this room, let’s make phone calls in the hallway.