Clava la piqueta a terra.

Here clava is the informal singular imperative of clavar.

So:

• clavar = to drive in, hammer in, stick in, nail
• clava = drive it in! / hammer it in!

For an English speaker, it may look like a normal present-tense form, because clava can also mean he/she puts in / drives in in other contexts. In this sentence, though, it is clearly a command.

Catalan usually does not need subject pronouns unless you want emphasis or contrast.

So the verb form already tells you who the command is aimed at:

• Clava la piqueta a terra. = Drive the peg into the ground.

You could say Tu, clava la piqueta a terra if you really want to stress you, but normally that would sound unnecessary.

Because Catalan normally uses an article before countable nouns in this kind of sentence.

• la piqueta = the peg / the stake

This usually means the speaker has a specific peg/stake in mind, or it is obvious from the situation which one is meant.

If you wanted an indefinite meaning, you would say:

• Clava una piqueta a terra. = Drive a peg into the ground.

Piqueta usually refers to a small stake, peg, or picket that can be driven into the ground.

The exact English word depends on context:

• in camping: peg
• in construction or marking ground: stake or picket

So even if the translation shown to the learner uses one specific English word, piqueta is a little broader and depends on the situation.

Not only.

Clavar can mean to nail, but more generally it means to drive in, stick in, hammer in, or embed firmly.

So in this sentence, it does not have to mean using a literal nail. It can simply mean:

• push it firmly into the ground
• hammer it into the ground
• drive it into the ground

That broader meaning is very common.

Here a terra means something like into the ground or to the ground.

Catalan often uses terra without an article when it means ground/earth in a general sense.

So:

• a terra = to/on the ground, down, into the ground
  depending on the verb

With clavar, the natural interpretation is into the ground.

If you translate word-for-word, it may feel strange, because English usually prefers into the ground, not just to ground. But in Catalan, a terra is very natural.

By itself, a terra can mean several related things:

• on the ground
• to the ground
• down
• into the ground

The exact meaning comes from the verb.

For example:

• caure a terra = to fall to the ground
• tirar una cosa a terra = to throw something onto the ground
• clavar una piqueta a terra = to drive a peg into the ground

So in your sentence, the verb clavar makes into the ground the best translation.

You change the imperative form of the verb:

• Clava la piqueta a terra. = informal singular (tu)
• Clavi la piqueta a terra. = formal singular (vostè)
• Claveu la piqueta a terra. = plural informal (vosaltres)
• Clavin la piqueta a terra. = plural formal (vostès)

So the rest of the sentence stays the same, and only the verb changes.

Yes.

If the object is already understood, you can use the weak pronoun la:

• Clava-la a terra. = Drive it into the ground.

Notice the spelling:

• imperative + pronoun
• connected with a hyphen

This is very common in Catalan commands.

A rough pronunciation for many learners is:

• KLAH-vuh luh pee-KEH-tuh uh TEH-rruh

In Central Catalan, a more IPA-like version is:

• [ˈklaβə lə piˈkɛtə ə ˈtɛrə]

A few useful notes:

• cl in clava sounds like English cl in clean
• v in many Catalan varieties sounds close to a soft b
• qu in piqueta gives a k sound
• the r in terra is tapped/trilled, not like a strong English r
• unstressed vowels are often reduced in Eastern/Central Catalan

The given order is the most neutral one:

• Clava la piqueta a terra.

That is:

• verb
• direct object
• place/direction

Catalan word order is somewhat flexible, but changing it usually adds emphasis or sounds less neutral. For example:

• A terra, clava la piqueta. = emphasizes into the ground
• Clava-la a terra. = uses a pronoun instead of the full noun phrase

So the original sentence is the best basic pattern to learn first.