Zet de fiets stevig vast aan het hek.

Because this is an imperative sentence: it gives a command or instruction.

• Zet = Put / fasten / place
• In informal singular Dutch, the imperative is usually just the verb stem.

So:

• zetten → stem zet
• Zet de fiets ... = Fasten the bike ...

If you wanted to include you, that would no longer be the usual command form.

The full verb is vastzetten.

Dutch has many separable verbs. In a main clause, the verb splits:

• vastzetten → zet ... vast

So in this sentence:

• Zet de fiets stevig vast aan het hek.

the parts are:

• zet = the conjugated part
• vast = the separable particle

Together they mean something like fasten / secure.

Because vastzetten is a separable verb, and in a normal main clause Dutch often separates the prefix-like part.

Compare:

• Ik zet de fiets vast.
• Zet de fiets vast.

But when the verb is not split, for example in the infinitive, it stays together:

• Je moet de fiets vastzetten.
• Het is belangrijk om de fiets vast te zetten.

This is a very common pattern in Dutch.

Stevig means something like firmly, securely, or tightly here.

It tells you how to fasten the bike.

So:

• de fiets vastzetten = to secure the bike
• de fiets stevig vastzetten = to secure the bike firmly / properly

Even though stevig is often translated as firm or sturdy, in this sentence it functions like an adverb in English: it describes the action.

Aan here means to or against, and in this context it is used for attaching or fastening something to something else.

So:

• aan het hek = to the fence

This is the normal Dutch preposition with verbs like vastmaken or vastzetten when something is being attached.

Because Dutch nouns have two main article types:

• de-words
• het-words

Here:

• de fiets is a de-word
• het hek is a het-word

Unfortunately, this usually has to be learned with each noun.

A useful thing to remember:

• diminutives always take het
• but otherwise article choice is often lexical and must be memorized

Not really. Here it is simply:

• aan = preposition
• het = definite article for hek

So:

• aan + het hek = aan het hek

This is just the preposition followed by the noun phrase het hek.

Do not confuse it with the different structure aan het + infinitive, as in:

• Ik ben aan het fietsen. = I am cycling.

That is a separate construction.

Dutch word order in commands often looks like this:

• Verb
• Object
• other information
• separable particle
• prepositional phrase

Here:

• Zet = verb
• de fiets = object
• stevig = adverb
• vast = separable particle
• aan het hek = prepositional phrase

So the sentence is very natural Dutch.

You may notice that vast comes fairly late. That is common with separable verbs.

Yes, absolutely.

• Zet de fiets vast aan het hek. = Secure the bike to the fence.
• Zet de fiets stevig vast aan het hek. = Secure the bike firmly to the fence.

Adding stevig just makes the instruction stronger or more specific.

On its own, zetten often means to put, to place, or to set.

But in combination with vast, the meaning changes:

• vastzetten = to fasten, to secure, sometimes to lock up

This is very common in Dutch: the particle changes the meaning of the base verb.

So you should learn vastzetten as a unit, not just translate it word-for-word as put fast.

Yes, that is also possible.

• vastmaken = to fasten
• vastzetten = to secure / fasten

Both can work in this context.

There can be small differences in style or nuance, but in everyday language both are natural. Vastzetten can sound especially appropriate when something is being fixed securely in place.

For a formal or polite command, Dutch can use u.

You might say:

• Zet u de fiets stevig vast aan het hek.

In practice, commands are often phrased differently in polite situations, for example:

• Wilt u de fiets stevig aan het hek vastzetten?

That sounds more polite, like Would you please secure the bike firmly to the fence?

A rough pronunciation guide is:

• Zet ≈ zet
• de ≈ duh
• fiets ≈ feets
• stevig ≈ STAY-vikh or STEH-vikh depending on accent
• vast ≈ vahst
• aan ≈ ahn
• het ≈ hut or very reduced
• hek ≈ heck

A rough full approximation:

Zet duh feets STAY-vikh vahst ahn hut heck

A few important notes:

• g in stevig is the Dutch guttural g/ch sound
• fiets has a long ie sound, like ee in see
• aan has a long aa sound