Translation:He is a slow dog, slower than a snail.
Yes, but these are two different kinds of langsamer. In ein langsamer Hund, -er is the nominative singular masculine ending. In langsamer als eine Schnecke, -er is the comparative ending. These two look the same but they are not. Indeed, the two are stacked together if the comparative form is used in a place where a nominative singular masculine form is needed. So “a slower dog” is ein langsamerer Hund.
Hope that helps.
The “object” of sein – as well as a small number of other words, most notably werden “to become” and heißen “to be called” – appears in nominative case.
If you’re interested in the theory as to why: The “object” of these verbs isn’t actually an object at all; it’s what linguists call a predicative: A phrase which is part of the verb, but refers back to either the subject or an object. Such predicatives often appear in the same case as whatever they are referring back to. For verbs like sein, werden, heißen, that thing is the subject, which means that the predicative appears in the nominative. (Side note: This is actually a common behaviour of predicatives, not only in German but also many other languages – until very recently including English. It’s the reason why grammar nazis will insist that you should say “it was I” instead of “it was me”.)
In this case, no. The use of “he” in English shows that we are talking about a particular dog (maybe the speaker’s pet) which is male, so er is the only possible translation. In general, there are three possibilities:
We are talking about a particular dog of known gender as above. In this case, use the appropriate pronoun for that gender, like in English.
We are talking about “a dog” of unspecified gender. In that case, use pronoun that corresponds to the grammatical gender of the word Hund, i.e. masculine.
We haven’t been talking about the dog before at all (maybe we answer the question “What is this?”). In this case, the pronoun is a general reference and you default to the neuter pronoun for introducing it: “Es ist ein Hund.” (if that sounds weird, you actually do the same in English. Compare how the after delivery a doctor would say “it’s a boy/girl” rather than “he/she’s a boy/girl”).
I hope that helps.
Would "Er ist ein langsamer Hund, langsam als eine Schnecke" be a correct translation ?
als = than in comparisons (langsamer als = slower than).
For comparisons of equality, you need wie: so langsam wie eine Schnecke = as slow as a snail.
langsam als would be like saying "as slow than".
And langsamer wie would be like saying "slower as". (Though lots of native German speakers do use wie in comparisons of inequality, that's not considered standard.)
A snail with a jetpack would trivially outpace many dogs. However, that's not interesting - finding a race where a snail would be faster than a dog under similar conditions is more interesting.
Well, terminal velocity, other things being equal, would be defined by sqrt(m/SC), with m for mass, S for projection area and C for how-aerodynamic-the-shape-is coefficient.
If we take some really big snail like Achatina achatina, we would be able to find a puppy of the same weight category, maybe even an adult dog of some toy breed.
However, due to snail's more aerodynamic form it would have a solid advantage. Different defensive reactions (flailing vs hiiding in a shell) push this difference even further.
Thus, in this situation, under equal conditions, I'd expect the snail to be faster than the dog.
(although they both would be better off as pets)