Learn Polish: Ten plan jest realny, o ile dotrzymają terminu. - This plan is realistic, provided that they meet the deadline.

While jeśli simply means "if", o ile is a restrictive conditional meaning "provided that" or "as long as". It strongly emphasizes that the plan is realistic only on the specific condition that the deadline is met. Also note that, just like before jeśli, you always need a comma before o ile.

Why do we use the future tense dotrzymają when the English translation uses the present tense "meet"?

English often uses the present tense for future conditions ("provided they meet it"). Polish is more strictly logical with its timeline: since meeting the deadline is an action that will happen in the future, Polish requires the future tense here. dotrzymają is the perfective future, emphasizing the successful completion of the action.

Why is termin in the Genitive case (terminu)? Shouldn't the deadline be a direct object in the Accusative case?

A handful of Polish verbs simply demand the Genitive case for their objects instead of the expected Accusative. dotrzymać (to keep or meet) is one of these verbs. Whether you are keeping a promise (dotrzymać słowa), a secret (dotrzymać tajemnicy), or a deadline (dotrzymać terminu), you must use the Genitive.

I usually think of "real" as prawdziwy. Is realny the standard word for "realistic"?