Identify the underlined words as a participial, gerund, or infinitive phrase. we couldn't open the locked door.

The underlined words here are "locked door". In this case, the answer would be PARTICIPIAL PHRASE. When we say participial phrase, this includes the participle and the modifiers. The participle can be in the present form and past form. The present form takes the -ing form, and the past form takes the -ed or the -en form. In general, the participle only functions as an adjective. The participle here is the word "locked" which serves as an adjective describing the noun "door".

Pardon me. to answer questions like this, we would have needed more information. for starters, two sentences from the excerpt that came with the question.