Mysterious usage of articles and singular/plural

Question

To the best of my knowledge, I believe the following four sentences are all grammatically correct, but why four different expressions for "dog"?
1. I like dogs;
2. Dog is a kind of animal;
3. The dog is the most lovely animal in the world.
4. A dog is able to run as soon as it's born.

Answer 1

1. I like dogs;

Discussing a subject, in general, in the plural form, doesn't have an article.

2. Dog is a kind of animal;

This is not really correct and probably shouldn't in the list. It might fit a different phrase like "Beef is a kind of meat."

3. The dog is the most lovely animal in the world.

Discussing a subject, in the abstract, and using the singular form, can have "the" as the article. It's similar to case #1. You can choose between #1 and #3.

4. A dog is able to run as soon as it's born.

Sometimes it's convenient to talk about a single instance of a class of things. "A car horn is able to beep." "A kangaroo is able to jump." The general abstract concept of kangaroos isn't able to jump, but a single one might be able to... so it sounds better to say "a kangaroo".

Answer 2

Sentence 1 is correct and grammatical.
Sentence 2 would be correct and grammatical and natural if it were "A dog is a kind of animal" or "Dogs are a kind of animal". As it stands, though, it's not a good sentence. Sentence 3 is correct and grammatical if you're referring to a specific dog. However, it would be better as "The (or That) dog is the loveliest animal in the world." It would not do to use this sentence to refer to dogs in general, though. Sentence 4 is grammatical as far as it goes, but it would be better as "Dogs are able to run as soon as they're born." However, sentence 4 is not true. Dogs are not able to run as soon as they're born. Horses and certain other animals are able to walk and even run or at least trot, shortly--within an hour, typically--after they're born. Whales are able to swim as soon as they're born.