1. Infinitive
1.1. after certain verbs
1.1.1. 1.forget, learn, help, train, teach
1.1.1.1. I forgot to save that file.
1.1.2. 2. expect, hope, need, choose, offer, want
1.1.2.1. You need to install a special application to help protect your data.
1.1.3. 3. agree, pretend, encourage, recommend, promise
1.1.3.1. I recommend you to use this OS.
1.1.4. 4. allow, decide,manage, mean, refuse, can afford
1.1.4.1. We can't decide which application to use.
1.2. after adjectives
1.2.1. 1. too+adjective
1.2.1.1. The computer was too slow to update that database.
1.2.2. 2. adjective+enough
1.2.2.1. We were good enough to persuade them to buy a computer.
2. I FORGOT TO SAVE a Word document.
3. -ing
3.1. after certain verbs
3.1.1. avoid, dislike, enjoy, finish, give up, mind
3.1.1.1. I finished programming a day ago.
3.2. after a preposition
3.2.1. I look forward to working in your company.
3.3. when the word is the subject
3.3.1. Programming is not good for our eyes.