Learn English – the difference between when, where, and which

There is no difference in meaning. There is a difference in use.

Relative clauses—the sort of clause you use, “which is blue” / “that is blue”, which tells us something more about the noun referred to by which or that—are of two sorts: restrictive and nonrestrictive.

A restrictive clause restricts the noun it modifies to what’s defined in the clause. The clause identifies the noun and is essential to your meaning. For instance:

I looked at the books which he sent me last week. … The books I’m talking about are the ones which he sent me last week.

A nonrestrictive clause adds information about the noun it modifies. The clause is almost parenthetical, it could be left out without changing your meaning:

I looked at the books, which he sent me last week. … The books have already been identified in our discourse, I’m just throwing in a by-the-way comment about when I got them.

Note the comma in that sentence: it sets the clause off and ‘marks’ it as something added. Now Which/That …

That may only be used at the head of a restrictive clause. It is not used, in any register, with a nonrestrictive clause.

Which may be used at the head of either sort of clause.

You may encounter another rule, which is loudly disputed. About a century ago the Fowler brothers suggested a “division of labour”—using that only at the head of restrictive clauses and which only at the head of nonrestrictive clauses. This proposal made sense to many people, it was picked up by several prestigious grammarians and style guides, and in consequence it’s often cited as a “rule”.

But the fact is, this division has never been generally adopted, and there’s no reason to follow it. I myself don’t follow it; quite the opposite, I employ “which” wherever I can, because I believe that that has entirely too much work to do already and a multiplicity of thats is likely to confuse the reader.

But that’s my choice. You’re free to follow your own rule, as long as you don’t put that at the head of a non-restrictive clause.

• That's the shop which sells elephants.
• This is an elephant which the shop sold ____ .

In these sentences, the relative clauses have been marked out in bold. Each relative clause is like a small sentence inside the bigger sentence. In these relative clauses, I used the pronoun which. I could use the pronoun that instead. They are both okay for this kind of sentence, where the relative clause is restrictive.

1. which sells elephants.

2. which the shop sold ________ .

In clause (1), which is the subject of the verb sells. In (2), which is the object. It has moved from that gap at the end. We can see that which is behaving like a Noun in these sentences. This is because which is a Noun! It's a special kind of Noun, a Pronoun. Whenever we need to replace a noun inside a relative clause we use which or that - or who if it is a person.

Now let's have a look at a different sentence:

• I play football in the park.

We can divide that sentence into different sections. The Subject is I. The Predicator (the verb) is play and the Object is football. But what about that funny bit at the end, in the park? What's that?

In the park here is an ADJUNCT. Adjuncts like this that get put at the end of sentences usually begin with a Preposition like in, on or at, for example. Adjuncts can also sometimes be Noun Phrases or Adverbs:

• I play football every day.
• I play football daily.

But most often Adjuncts like this begin with a preposition. Adjuncts usually tell us information about why, when, how, who with or where something happened. That kind of thing. This information is always extra information. It isn't part of the essential grammar of the sentence. I play football still makes sense without an adjunct at the end.

The relative word where doesn't usually replace a noun! It replaces an adjunct or a preposition phrase - ones that tell us about where. (Remember that an adjunct will normally include a preposition and a noun). This also means it can't replace the Subject or Direct Object of a verb. It also can't usually replace a noun which is the object of a preposition (although remember that preposition phrases can also be the complements of prepositions). Where nearly always replaces a preposition phrase or an adjunct.

In the original Poster's sentence

• London Bridge is one of the most popular places wwww people want to visit ____.

We can see that that gap at the end there with the missing bit, is the Direct Object of the verb visit. We don't expect to see a preposition after visit either. This word can't be an adjunct or a preposition phrase. It looks like it must be a Noun Phrase. Therefore, we need to use which or that. A good rule-of-thumb test to see if it should be which or where is to see if you could put the word it or them in the gap. If you can, you can use which:

• London Bridge is one of the most popular places which /people want to visit it/.

If you can't but you could put the word there in the gap, then you should use where:

• ... the house where /I used to live there/

Edit note

For some speakers, this rule -that where can't replace direct objects doesn't seem to be so strict. See F.E.'s comment about his variety of English. For my dialect - I'm a British English speaker - this rule is quite strict.

Hope this helps!