It, there, and one look like ordinary pronouns, but they often don't point at anything. Their job is structural — they fill a slot so the sentence has a subject or so a noun doesn't have to be repeated.
An English sentence (outside of commands) must have a subject. When there's no real subject to put in that position, English uses it or there as a placeholder. And when a noun has already been mentioned and you don't want to say it twice, one or ones stands in for it.
Referential it vs dummy it
The most familiar it points back to something already mentioned. This is referential it — it has a clear antecedent, usually a thing, animal, or situation.
Dummy it is different. It doesn't refer to anything at all. It sits in the subject position because the grammar needs a subject, but there's no real topic behind it.
In the first sentence, it is not the rain, the weather, or the sky — it's simply a placeholder. This is the defining feature: dummy it has no antecedent you can point to.
Dummy it — four main uses
1. Weather and nature
Any description of weather, temperature, light, or general outdoor conditions uses dummy it.
You cannot drop the it. "Is raining" or "Was windy yesterday" is ungrammatical in English, even though many languages allow it.
2. Time, dates, and days
Again, it doesn't refer to the clock or the calendar — it's just a subject slot.
3. Distance and temperature
4. Anticipatory it (long subjects moved to the end)
When the real subject is a long phrase — an infinitive or a that-clause — English prefers to move it to the end of the sentence and put it in the front slot.
The alternative — "To arrive on time is important" — is grammatically correct but sounds heavy and old-fashioned. Modern English almost always uses the anticipatory form.
Substitute one and ones
When a countable noun has already been mentioned, you can use one (singular) or ones (plural) instead of repeating it.
Without the substitute, you'd have to say "the blue cup" and "new shoes" — correct, but repetitive when the noun is obvious from context.
Where it appears
After an adjective. The most common position.
After this, that, which, each.
After the.
One replaces only countable nouns
This is the key limit. One and ones can stand in only for nouns you can count. With uncountable nouns — information, advice, water, music, furniture — substitution doesn't work.
For uncountable nouns, either repeat the noun or rephrase.
These, those — no "ones" after them
A common slip: after these and those, English doesn't add ones. The plural is already built in.
A note on generic one
There's another one in English — the formal generic "one should always be polite", meaning people in general. That's a different word with a different job, and it's not what this lesson covers.
There — the existence marker
There is the third placeholder. Its job is to announce that something exists or is located somewhere. Grammatically it's a dummy subject, not a place word.
The real subject — a coffee shop, twenty people, a problem — comes after the verb. The verb agrees with that real subject, not with there: there is a cat, there are two cats. This construction has its own rules around agreement, articles, and tense, and is covered separately.
Don't confuse this there with the place adverb there meaning "in that place" ("Put it there"). They look identical but do different jobs.
It vs there vs one — the one-paragraph summary
Three small words, three distinct jobs. It fills the subject slot when the sentence has no real topic — weather, time, distance, or a long subject pushed to the end. There announces that something exists or is present somewhere, with the real subject coming after the verb. One (and ones) replaces a countable noun that has already been mentioned, so you don't have to repeat it. None of them carries its literal meaning in these uses; all three are structural.
Common mistakes
Dropping dummy it
Confusing it and there
One with uncountable nouns
Ones after these/those
Forgetting plural agreement with there