Arancha sent me a great question last night:
Cual de las siguiente frases es la correcta en inglés:
1ª/ I THINK THAT TO BUY IN THE SUPERMARKET........
2ª/ I THINK THAT BUYING IN THE SUPERMARKET.........
3ª/ I THINK THAT BUY IN THE SUPERMARKET.........
Do you know the answer? It's #2, and here's the reason why.
If you look at the way the verbs are organized, you would probably have a sentence like this:
PART ONE: >>I think that<<
PART TWO: >>buying beer in the supermarket is less expensive than buying beer in the corner store.<<
The sentence has TWO verbs -- THINK and IS. Both of them have different subjects: I think and buying in the supermarket is. What happens is that the information that comes after "that" has to be a noun - you always think of something that needs a verb, and that verb always needs another subject. Here are some more examples:
I think that McDonald's is delicious. (Me: "I think that McDonald's is delicious!")
He thinks that his mother is going crazy. (Jim: "I think that my mother is going crazy.")
We think that we will probably go to France this summer. (Alberto and I: "We will probably go to France this summer.")
They think that Barcelona is a very expensive city. (Jon and Ann: "Barcelona is a very expensive city!")
Why? In all of the examples, "I think that" almost works like reported speech (estilo indirecto):
º I think that McDonald's is delicious.
º He told his mother that McDonald's is delicious.
º The advertisements say that McDonald's is delicious.
º It has been reported in EL PAÍS that McDonald's is delicious.
In Spanish, you would need the infinitive, but in English, we treat it like a noun idea, so we use the gerund form of the verb (since it's usually the easiest noun form.)
This is one really excellent reason why an English-only dictionary is a much better idea than a translating dictionary: most of them will tell you when you use words like THAT to connect secondary ideas with verbs like say, think, know, expect, tell and explain.