De slang ligt in de tuin.

Because slang and tuin are both de-words in Dutch. Dutch nouns take either de or het in the singular, and this is something you usually have to learn with each noun.

• de slang = the snake
• de tuin = the garden

In the plural, both de-words and het-words take de, so:

• de slangen = the snakes
• de tuinen = the gardens

Ligt is the present tense of liggen, which literally means to lie.

Dutch often uses position verbs where English often just uses to be. So instead of simply saying something is somewhere, Dutch often says whether it is:

• liggen = lying / being in a horizontal position
• staan = standing
• zitten = sitting / being located in, on, or attached in some contexts

A snake naturally has a lying, stretched-out position, so liggen is very natural here.

So De slang ligt in de tuin sounds more natural in Dutch than De slang is in de tuin.

Grammatically, people would understand it, but it is less natural in ordinary Dutch.

Dutch strongly prefers a position verb when talking about where something or someone is. Since a snake is typically thought of as lying on the ground, ligt is the usual choice.

So:

• De slang ligt in de tuin. = natural
• De slang is in de tuin. = understandable, but less idiomatic

Because Dutch main clauses usually follow the verb-second rule.

That means the finite verb goes in the second position of the sentence. Here:

1. De slang
2. ligt
3. in de tuin

This is very typical Dutch word order.

You can also move another element to the front, but then the verb still stays second:

• In de tuin ligt de slang.

That is also correct, but it puts more emphasis on in the garden.

Because Dutch uses in for being inside an area or enclosed space, and a garden is usually treated as an area you are in.

So:

• in de tuin = in the garden

Using op would usually sound wrong here. Op is used more for surfaces or places thought of as a platform/area in a different way, such as:

• op tafel = on the table
• op straat = in the street / on the street

For a garden, in is the normal preposition.

De means the, so it refers to a specific snake and a specific garden.

• De slang ligt in de tuin. = a particular snake is in the garden
• Een slang ligt in de tuin. = a snake is in the garden

A learner often needs to notice that Dutch articles work much like English articles here:

• de / het = the
• een = a / an

Ligt is the third-person singular present tense form of liggen.

The verb is conjugated like this:

• ik lig = I lie / am lying
• jij ligt = you lie / are lying
• hij/zij/het ligt = he/she/it lies / is lying
• wij liggen = we lie / are lying
• jullie liggen = you all lie / are lying
• zij liggen = they lie / are lying

Since de slang is singular, Dutch uses ligt.

Yes, it can definitely catch English speakers out.

In this sentence, slang means snake. That is an ordinary Dutch noun:

• de slang = the snake

Dutch can also use slang to mean slang in the language sense, but in De slang ligt in de tuin, the meaning is clearly snake, because informal language does not usually lie in the garden.

So context matters.

The hardest part for many English speakers is ui.

• tuin sounds roughly like t-ow/oi-n, but there is no exact English equivalent.
• The Dutch ui is a special diphthong that often takes practice.

A rough pronunciation guide for the whole sentence might be:

• de ≈ duh
• slang ≈ slahng
• ligt ≈ likht / likt with a throaty Dutch g/ch
• in = in
• de ≈ duh
• tuin ≈ t-ow/oi-n

The pronunciation of ui is one of the classic difficult Dutch sounds for English speakers.

The plural would be:

De slangen liggen in de tuin.

Notice two changes:

• slang → slangen
• ligt → liggen

So:

• singular: De slang ligt in de tuin.
• plural: De slangen liggen in de tuin.

The past tense of liggen here is lag:

• De slang lag in de tuin. = The snake was lying in the garden.

And the plural would be:

• De slangen lagen in de tuin. = The snakes were lying in the garden.

This is a useful verb to learn early, because liggen, staan, and zitten are very common in Dutch.