Giải bài tập

1) We tried to open the window, but it was so hot outside it didn't help.
- "Tried to" is used when indicating an attempt to do something that may require effort or action.

2) He tried to get a job in a newspaper firm but they wouldn't hire him.
- Similar to the previous example, "to get" indicates his attempt to achieve a certain outcome.

3) He tried getting a job in a newspaper firm but he still wasn't satisfied.
- "Getting" can imply a continuous effort or action, suggesting he was involved in the process of obtaining a job.

4) You should stop smoking, it's not good for your health.
- "Stop smoking" is the correct phrase, as "stop" is followed by a gerund (verb+ing) to indicate the action that should be ceased.

5) We stopped studying because we were tired.
- Like "stop," "stopped" is also followed by a gerund to indicate the conclusion of an action.

6) They will stop having lunch at twelve.
- "Stop having" implies that they will cease this action at a specified time.

7) We stopped to have lunch because we were really sleepy.
- This indicates a change of action; they stopped one activity in order to start having lunch.

8) Oh no! I forgot to buy milk.
- After "forgot," we use "to" to express the forgotten action that someone intended to take.

9) Please don't forget to pick up some juice on your way home.
- Again, "forget" is followed by "to" to express an intended action that needs to be remembered.

10) I forget to lock the door, but I'm sure I must have locked it.
- The phrase indicates an action that was supposed to be done but didn't happen.

11) Have we studied this before? I've forgotten to learn it.
- This shows that the learning action is something they intended to do but forgot about.

12) Please remember to bring your homework.
- "Remember to" indicates the action that the person should keep in mind to do later.

13) I remember going to the beach as a child.
- "Remember" can be followed by a gerund when you're recalling a past action.

14) Finally I remembered to bring your book! Here it is.
- This structure indicates a past action that was successfully completed.

15) Do you remember eating steak in that little restaurant in Rome?
- The gerund form "eating" is used here because it relates to a memory of a past event.

16) I regret to tell you the train has been delayed.
- "Regret to" is used when one has to announce bad news or something unfortunate.

17) I regret telling Julie my secret. Now she has told everyone.
- "Regret telling" reflects on a previous action and its negative consequences.

In summary, the answers given are focused on correctly applying the grammatical rules regarding the usage of infinitives (to + verb) and gerunds (verb + ing) based on the context of each sentence.