it只能指代不可数名词吗

不是,it的用法很多的.
一、It用作实词
表达以下概念:指代前文提到的事物,前文中的this, that；替代前文中的内容；指代一位性别不明的小孩或未知的人；指代未指明但谈话双方都知道的那件事；指代时间、天气、气候、距离等自然现象……
二、It用作形式主语
替代作主语的从句、动词不定式、动名词,而把真正作主语的从句、动词不定式、动名词置于句尾.
It 作形式主语的常见句型：
1. 代作主语的动词不定式,其句型为
(1) It be adj. (for sb.) to do sth.
此处adj. 通常为描述事件的形容词：easy, difficult, hard, necessary, unnecessary, possible, impossible, likely, unlikely, right, wrong, important, unimportant, legal, illegal, well-mannered, ill-mannered, polite, impolite, clear, obvious, certain, suitable, proper, fit, useful, useless, dangerous…
例 It is illegal (for a teenager) to drive a car without a license.
(2) It be adj. of sb. to do sth.
此处adj. 通常为描述人的形容词：kind, unkind, nice, rude, cruel, considerate, thoughtful, thoughtless, careful, careless, silly, foolish, stupid, clever, wise, crazy.
例 It‘s kind of you to help me with the problem.
(3) It替代作主语的动名词的常见句型
It‘s no good/use doing…
It‘s (well)worth doing…
It‘s (well)worth one‘s while doing/to do…
It‘s (well)worth while doing/ to do
例 It‘s no use crying over spilt milk.
2. It替代作主语的从句常见句型
(1) It is + noun +从句
例 It is no secret that the president wants to have a second term at office.
(2) It is adj. +clause
It‘s surprising that… (should)………竟然……
It‘s a pity/shame that…(should)………竟然……
例 It‘s important that you should apologize to her for your rudeness.(=It‘s of much importance that you should apologize to her for your rudeness.)
(3) It verb sb. + clause= It is v-ing + clause
It+surprise/delight/interest/disappoint/worry/disturb/annoy/amaze
/bother/concern/frighten/please/anger sb. that…
例 It worried me that she drove so fast.(= It was worrying that she drove so fast.)
(4) It verb (to sb.) that…= sb/sth verb to do
(verb = appear, seem, come about, emerge, follow, chance, happen, occur, transpire, turn out , work out)
例 It (so) happened/chanced that they were out.(= They happened/chanced to be out.)
(5) It is v-ed that…=sb/sth is to do
(verb=say, report, think, believe, hope, expect, agree, accept, decide, determine, intend, plan, understand, know）
例 It is said that the couple have gotten divorced.(=The couple are said to have gotten divorced.)
(6) It is v-ed that … (should)…
(verb=demand, request, require, order, suggest, advise, recommend
例 It is suggested that they should begin with the third question.
三、It作主语的句型
1. It takes sb. … to do…(=sb takes…to do…)某人用多长时间做某事
例 It took the men a week to mend our roof.(= The men took a week to mend our roof.)
2. It‘s (just)(un)like sb. to do…（不）像某人做某事的风格
例 It was (just) like him to think of helping us.
3. It‘s (about/high) time that… should /v-ed…是该做某事的时候了
例 It‘s(about/high) time that we should take action.
4. It‘s the x-th time (that) … have v-ed…第几次做某事了
例 It‘s the third time that he has failed the driving test.
5. It is/has been… since …continuous v-ed(延续性动词) 某动作已有多长时间不发生了
例 It‘s 10 years that he lived here
6. It was(not)… before…过(不)了多长时间某动作发生了
例 It was not long before they arrived.
四、It 作形式宾语
用来替代作宾语的从句、动词不定式、动名词,而把真正作宾语的从句、动词不定式、动名词置于句尾.
It 作形式宾语的常见句型：
1. verb+ it+ adj./noun (for/of) to do/clause (verb=think, believe, suppose, consider, feel, make, keep…)
例 I think it hard for you to do the task on your own./I think it hard that you‘ll do the task on your own.
2. verb+it+adj./noun (one‘s) doing (adj.=useless/worth/worthwhile)(noun=no use/no good/worth one‘s while/a waste of time/money/energy/words) (verb=think, believe, suppose, consider, feel, make, keep…)
例 I‘ll make it worth your while telling me about his secret.
3. verb+it+ important/unimportant/necessary/unnecessary/natural/essential that … (should)…
verb+it+of much/great/no/little importance that…(should)…
(verb=think, believe, suppose, consider, feel, make, keep…)
例 I think it important that you (should) attendthe conference.
4. verb + it+ as+ noun/adj.+ clause (verb=accept, regard, take, see, view)
例 The lecturer takes it as encouraging when so many students attend his lecture.
5. v. +it + prep. + that…
owe it to sb. that…把…归功于…
leave it to sb that…把…留给某人去做
take it for granted that …想当然
keep it in mind that…
例 Don‘t bother to arrange anything. Just leave it to me to sort out.
6. It用在不能直接跟宾语从句的动词后面,尤其是表示好恶的动词后,enjoy, like, love, dislike, resent, hate, don‘t mind, be fond of, feel like, see to 宾语从句紧跟it之后
例 I hate it you can swim so well and I can‘t.
7. It用在不能直接跟宾语从句的介词后面,宾语从句紧跟it之后（except that例外）
例 I‘m for it that you will follow their advice.
五、强调句型
It is/was+被强调部分+ that(who)… 强调句型用来强调谓语动词以外的任何句子成分.当被强调部分是人时也可以用who.
在使用强调句型时需注意以下几点：
1. 请注意强调句型的特殊疑问句
例 When was it that he changed his mind to take part in the activity?
2. 在强调原因状语从句时,只能强调由because所引导的从句
例 It was because he was ill that he didn‘t come to school yesterday.
3. 在强调not … until结构时必须把not与until一起放到被强调的位置上
例 It was not until she took off her dark glasses that I realized she was my brother.
4. 注意强调句型与定语从句的区别
例 It was at 7 o‘clock that he came here yesterday.( 强调句型)
It was 7 o‘clock when he came here yesterday.(定语从句)

it可以用作人称代词，它还可以指时间、天气、距离、上下文等，另外，it还可以作先行词，作形式主语和形式宾语。
2. 用法指南
(1)作为人称代词，it可以用来指东西、动物、婴儿和未确定身份的人。
Where's my book? Have you seen it?
我的书在哪里？你看见了没有？
The dog is in the gard...

不是，一般情况下指人以外的一切生物和事物。有时也指婴儿、小孩和不知性别的人。还可指代上文提到的事情，或者表示天气、季节、温度、距离、时间或事物的状态等。既包括可数名词也包括不可数名词

不是的。也可指代可数名词，但指代时该物体是单数。

不是，it还可以指代可数名词的单数。另外，它还可以用作形式主语，形式宾语。还可以用在特定句型当中，比如强调句。