Could you tell me which of the phrases below is correct? Or if there are some of them, is there any difference between them?
"to walk along the streets";
"to walk in the streets";
"to walk on the streets";
or "to walk the streets".
Here's my Pennsylvanian American English take:
He was walking along the street. - He was walking next to, beside, at the side of, (on the sidewalk of) the street. The focus is on his activity- walking, and his location- the side of the street, without much reference to purpose or destination.
He was walking in the street. - He left the side of the street, or the sidewalk, is was walking in the middle of the street, on the part of the street usually reserved for vehicular traffic.
He was walking on the street. - This is not as likely to be heard in my part of the world, but could mean the same as walking in the street.
On the other hand, He was walking on a particular named street, is a normal thing to say and hear. He was walking on Main Street is the way to state the location of his activity.
He was walking the street.- This could be heard in some contexts, probably when talking about a regular walk he does for some purpose. The policeman was walking the street (because that's what he usually does as part of his purposeful job), when he witnessed the robbery.
The most common variation of "to walk _______ the street" in my speech is "to walk down (or up) the street".
He was walking down the street.- He was walking. The street is the location of his walking. Down the street carries some implication of purpose (or at least direction). He was walking on a particular street to get from one place to another. Depending on the speaker's sense of map orientation or elevation or even nothing at all, he might say "up the street" or "down the street" with little or no significant difference of meaning.
To use your phrases exactly in context:
I like to walk along the streets of Philadelphia. - When someone says this, "along" for me brings the focus closer to what I might see as I walk on the sidewalk of streets in the city.
I like to walk the streets of Philadelphia. - When someone says this, I picture a map of the various streets of the city that I might explore on foot.
The differences in these first two are subtle and maybe personal, so the sentences say basically the same thing.
I like to walk in the streets of Philadelphia. - Here, again, I picture someone leaving the sidewalk and walking on the actual street where cars normally go.
I like to walk on the streets of Philadelphia.- This could be used with the same meaning of any of the previous three, but would be less likely to be heard or said, maybe because it's not clear whether the speaker means "in the streets" or "along the streets".
I like to walk down the streets of Philadelphia. - I picture using the street to get from one place to another; the walking probably has (but doesn't have to have) a destination or purpose.
Sorry for the length. I didn't set out to write so much, but your questions make me think about my language, and it's fun. Thanks.