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First, let's get our vocabulary right. The verb should be unlock, not open.
This is a great point that you've brought up, Mahdi.
The phrase a/the key to ... has both a literal meaning and a figurative meaning. You've cited a sentence with the literal meaning. When a key to ... is used literally, we explain its function with an infinitive verb as shown in your sentence:
This is a key to unlock the door.
But when we use a key to ... in its figurative meaning (check out http://www.ldoceonline.com/dictionary/key_2, meaning no. 2, to see what this is), we can follow the phrase with a noun or noun phrase
Here is the key to a good marriage.
or with a gerund noun phrase
Here is the key to living a rich, full life.
I hope that stops any confusion, Mahdi. Thanks for bringing this point up!
We have some verbs like want that require a following verb to be in the infinitive form, Izzy, and to check out whether or not an infinitive is required, you can use the question "What?" For example:
A: What do you want? B: I want to learn English.
Notice that there's no preposition following want in the question. That's very important to keep in mind.
We also have lots of verbs with allied prepositions, Izzy. That means these verbs always go together with their allied prepositions. Some examples are devote oneself to, interest somebody in, arrest somebody for, etc.
For this answer, though, I want to focus on how those verbs are transformed into past participle adjectives and used together with their allied prepositions or are used in the passive voice since these are the more typical ways they're used: be devoted to, be interested in, be arrested for.
A: What are you devoted to? B: I'm devoted to making my neighborhood a safe place to live in.
A: What is she interested in? B: She's interested in reducing environmental pollution.
A: What was he arrested for? B: He was arrested for robbing a bank.
Notice that each question ends with an allied preposition. Whenever we have a past participle adjective or passive voice form with an allied preposition, we need to use either a noun, a noun phrase, or an -ing verb form after it as you see above.
So that's why the phrase ... which are devoted to comment ... is wrong. Since be devoted to has a past participle adjective with an allied preposition, we need to put the verb comment in the -ing form and say ... which are devoted to commenting ....
Finally, I wish there were a hard-and-fast rule to give you, but there isn't one. You just need to memorize which prepositions are allied with which past participle adjectives or passive voice forms. But use that question beginning with What ...? to help you figure out whether an infinitive is needed in the answer or not.
Yes, it would be, Izzy. As long as the question beginning with what doesn't end with a preposition, you know that the answer/statement is going to require an infinitive verb.
Here's are more examples:
A: What did she tell you? B: She told me to clean up my room.
A: I know they're going to the factory to apply for jobs, but what do they expect? B: They expect to get hired.
Now compare those to these:
A: What are you so excited about? B: I'm excited about getting a promotion!
A: What was he indicted for? B: He was indicted for bribing an officer of the court.