aback, a back

Aback or aback ( one word ) , most commonly used in the phrase "taken aback", means surprised or disconcerted--
He was taken aback when he heard the news.
I though he knew about the accident, but he seemed taken aback when I mentioned it.

Aback is also a nautical term, meaning with the wind against the forward side of the sail or sails--
The schooner was caught aback, and arrived later than expected.

A back or a back ( two words ) describes one ( a ) rear part, or part farthest from the front or main part ( back )--
He had a back injury a few years ago, and it still bothers him.
Should we get on the highway, or take a back road?
There is a back room to this place; it was once a speakeasy.

A back may also mean one ( a ) large tub or vat ( back )--
The cloth will have to soak in a back for at least an hour.

A back may also means one ( a ) ferryboat hauled across the water by a chain ( back )--
There is a back you can used to get across the river, about a mile north of here.

Now that you know that, you can say--
"I knew the place had a back porch, but I was taken aback when I saw the size of it."