Hunden går i koppel.

Hund means dog, while hunden means the dog.

In Swedish, the definite article is often added as an ending:

• en hund = a dog
• hunden = the dog

Since hund is an en-word, its definite singular form ends in -en.

Går is the present tense of gå.

The basic meaning of gå is go or walk, so går literally means goes/walks. In this sentence, though, it is part of a very natural Swedish expression, so the whole sentence is understood as something like the dog walks on a leash or the dog is on a leash.

Because i koppel is the standard Swedish expression for on a leash.

This is one of those cases where Swedish and English use different prepositions. English says on a leash, but Swedish says i koppel.

• i koppel = the normal idiomatic expression
• med koppel = literally with a leash, which sounds less natural if you mean the dog is being kept on a leash
• på koppel is not the normal expression here

Because i koppel is a fixed expression, and Swedish often leaves out the article in this kind of phrase.

So even though the noun is:

• ett koppel = a leash
• kopplet = the leash

the idiomatic phrase is simply i koppel, not i ett koppel.

Here, koppel is being used in a general sense: the dog is in a leashed state, not specifically being described in relation to one particular leash.

Yes, går is present tense.

Swedish present tense can correspond to both:

• walks
• is walking

So depending on context, Hunden går i koppel could be translated in slightly different ways in English. Swedish does not always make the same simple-present vs. progressive distinction that English does.

Yes, absolutely.

Hunden är kopplad means the dog is leashed or the dog is attached to a leash. That is also very natural Swedish.

The difference is mainly one of wording:

• Hunden går i koppel = the dog goes/walks on a leash; this is a common expression about how the dog is being walked or kept
• Hunden är kopplad = the dog is leashed

Both are correct, but they are not exactly the same phrase.

The sentence follows normal Swedish main-clause word order:

• Hunden = subject
• går = verb
• i koppel = prepositional phrase

So the pattern is:

subject + verb + adverbial/prepositional phrase

A useful thing to know is that Swedish main clauses normally keep the verb in the second position. So if you start with something else, the verb still comes second:

• I parken går hunden i koppel. = In the park, the dog is on a leash.

You can change the noun like this:

• Hundarna går i koppel. = The dogs are on leashes / The dogs walk on leashes
• Hundar går i koppel. = Dogs are kept on leashes / Dogs walk on leashes

So:

• hund = dog
• hunden = the dog
• hundar = dogs
• hundarna = the dogs

Very roughly, you can think of it like this:

• Hunden ≈ HUN-den
• går ≈ gor with a long vowel
• i ≈ ee
• koppel ≈ KOP-pel

A few notes:

• The å in går is not exactly like any single English sound, but or in more is a rough starting point.
• The u in hunden is also not exactly like English u.
• The main stress is on the first syllable of hunden and koppel.

So the whole sentence is roughly:

HUN-den gor ee KOP-pel

But like most pronunciation guides based on English spelling, that is only approximate.