I often noticed him TO leave school alone. - 我经常看到他独自离开学校。
TO preposition (INFINITIVE) 不定式 strong /tuː/ weak /tʊ/ /tu/ /tə/ US/tə/ US/t̬ə/ US/tu/
› used before a verb to show that it is in the infinitive（用于动词前，表示不定式）
› used after some verbs, especially when the action described in the infinitive will happen later
（用于某些动词后，尤其是不定式所描述的动作将会随后发生时）
She agreed to help. 她答应帮忙。
I'll have to tell him. 我得告诉他。
Sadly she didn't live to see her grandchildren.
令人伤心的是，她没有活到孙辈出生的那一天。
› used after many verbs of agreeing, needing and wanting
（用于许多表示赞成、需要和想要的动词后）
I need to eat something first. 我首先得吃点东西。
I'd love to live in New York. 我想要住在纽约。
That child ought to be in bed. 那个孩子应该上床睡觉。
› used instead of repeating a verb clause （用于代替动词分句，以免重复）
"Are you going tonight?" "I'm certainly hoping to." “你今晚要走吗？”“我当然希望走。”
› used in phrases where there are reported orders and requests
（用于表示间接命令、要求的短语中）
He told me to wait. 他叫我等。
Did anyone ask Daniel to book the room? 有人要丹尼尔订房间了吗？
› used after some adjectives （用于某些形容词后）
It's not likely to happen. 这事不大可能发生。
Three months is too long to wait. 等3个月太久了。
She's not strong enough to go walking up mountains. 她身体不够壮，登不上山。
› used after some nouns （用于某些名词后）
He has this enviable ability to ignore everything that's unpleasant inlife.
他有这种令人羡慕的本事，能够不去理睬生活中的种种不快。
This will be my second attempt to make flaky pastry. 这将是我第二次尝试制作薄馅饼皮。
› A clause containing to + infinitive can be used as the subject of a sentence
（含有带 to 的不定式的分句可用作句子主语）
To go overseas on your own is very brave. 只身去国外，真勇敢。
My plan was to get it all arranged before I told anyone.
我的计划是先把一切都安排好再告诉别人。
› used after question words （用于疑问词后）
I don't know what to do. 我不知道该怎么办。
Can you tell me how to get there? 你能告诉我怎样去那儿吗？
› used with an infinitive to express use or purpose （与不定式连用，表示用途或目的）
I'm going there to see my sister. 我要去那儿看望我的姐姐。
This tool is used to make holes in leather. 这个工具是用来在皮革上打眼的。
To make this cake, you'll need 2 eggs, 175 g sugar and 175 g flour.
做这个蛋糕，你需要两个鸡蛋、175克糖和175克面粉。
He works to get paid, not because he enjoys it. 他工作是为了挣钱，而不是因为喜欢工作。
› You can introduce a clause with a phrase containing to +infinitive
（含有带 to 的不定式的短语可引出分句）
To be honest (= Speaking honestly), Elaine, I prefer you in the greyshirt.
说实话，伊莱恩，我喜欢你穿灰色的衬衫。
To be quite truthful with you, Betty, I never really liked the man.
坦率地跟你说，贝蒂，我从未真正地喜欢过这个男人。
› used with an infinitive after 'there is' or 'there are' and a noun
（与不定式连用，用于 there be + 名词这种结构后）
There's an awful lot of work to be done. 有许许多多的工作要做。
to be going on with UK› To be going on with means in order to continue with the presentactivity or situation 为了继续干下去
Do we have enough paint to be going on with, or should I get some more while I'm out?
继续干下去我们的油漆够用吗，我是不是该外出时再买一些？
追问可是不是有一个固定搭配是notice sb do sth吗？为什么这里又要to do？
来自：求助得到的回答