I've taken a picture of a cow. Someone asked me: "did you take a picture of cow?" (he wasn't sure what animal it is). Should I reply "I think so it was cow" or "I think so it was a cow"? (my answer is to confirm his guess about kind of animal)
Of course you may use "I think so; it was a cow." It can be any cow. Using "cow" without an article will make it sound like it's a person you are referring to (as "cow").
Why not this:
Yes, it was a cow.
Saying "I think so" at the beginning of the sentence makes it sound like you are unsure of what kind of animal it was. If you are unsure, you would say:
I think it was a cow.
I think so – I think it was a cow.
However, if you're completely certain that it was a cow, you would simply say:
Yes, it was a cow.
As Shaona mentioned, you need to include the article a in your sentence. You can't simply say, "It was cow." That part is tricky, though, because some animals don't require an article, particularly in the plural:
We saw deer in the park.
We saw geese on the lake.
We took pictures of cows yesterday.
However, in the singular:
We saw a deer in the park.
We saw a goose on the lake.
We took a picture of a cow yesterday.
I don't think it's all that tricky. Plurals NEVER use an article. "We saw cows." Singulars require either an article or an adjective indicating "one" or identifying the individual one. "We saw a cow." "We saw the cow." "We saw one cow." "We saw Farmer Brown's cow."– JayJul 5, 2013 at 16:10
1@Jay: By "tricky" here, I meant, "I can see why a non-native might think there wouldn't be an article required." As for your assertion that plurals "never" use an article, I can't quite agree with that: "Did anybody see the ducklings crossing the street?" "Yes, we saw the ducklings crossing the street." "The geese were flying in a V-formation." "The conservationist released the fish back into the wild." Those the articles aren't required, but they're not incorrect, either. Jul 5, 2013 at 16:22
See also: "We saw the cows". Jul 5, 2013 at 16:25
@JR RE "We saw the cows." Good point. I retract my statement that plurals never use an article. That's just wrong.– JayJul 8, 2013 at 21:55
1@Jay: I think if your comment had said, "Plurals don't require an article," you would have been on the mark. You were on the right track, I think. Jul 8, 2013 at 23:33