何時在句子中使用to

Most schools require students to wear uniforms. ~ correct.
format: require soneone to do something
e.g. The agreement requires all parties to renounce the use of violence.

大多數 infinitive 要加 to, 倒不如問哪一些動詞 infinitive without to

只有infinitive without to (bare infinitive) 不用加 to. 下列例子

1. Let us pray. (不是 Let us to pray)
2. I saw a young mother slap her child. (bare infinitive.)
3. Last Sunday I heard them play the symphony. (bare infinitive ~ I heard the whole symphony.)
4. He made her fill out the form. (bare infinitive)
5. I shall have the porter bring out the bags. (bare infinitive)
6. Do you feel the earth move? (bare infinitive)
7. He had better go home. (bare infinitive)

8. She helps (to) clean the machine. (to 可有可無)
9. I have never known him (to) pay for the drink. (to 可有可無)
10. All I did was (to) give him a little push. . (to 可有可無)

Do is sometimes used for emphasis.
11. We don’t need advice, but we do need money.
12. I did enjoy that dinner. It was delicious.

What Ryan did helped save our company.

What Ryan did helped (to) save our company.
"What Ryan did" is a noun phrase. This noun phrase is the subject of the sentence.
"helped" is the main verb of the whole sentence. (past tense)
其實應該是 to save, an infinitive, 不過句子用 infinitive without to. ~ similar to sentence 8. 變成 helped save.
所以三個動詞表面看,連在一起

小心: 以下句子grammatically OK, meaning 不同)
I saw a young mother slapping her child. (She slapped him several times)
I saw a young mother slap her child. (She slapped once.)

As I walked past the church I heard them playing the symphony.
(The people were in the middle of playing when I walked past.) 比較句子3

參加者需年滿十八歲
It is required that participant should be 18 years of age or older.
It is required that participant should reach the age of 18.
It is required that minimum age of participant should be 18.

Like going to Hongkong .

Miner's has no money in going to Beijing .

You might get the answer from here:

http://www.thefreedictionary.com/to

did helped - here the "did" is a helping verb , to give emphasis to the verb"helped"

What Ryan really helped save our company?