"Sorry for interrupting". No other form of the verb is possible. Here's an important rule for you: Prepositions - e.g. for, of, from, by, on, with, at, from - should always be following by the -ing form. You can never have a base form or conjugated past or present tense of a verb after these words. (The only exception to this rule is the preposition 'to', which is often (but not always) followed by the infinitive).