Las piezas forman un círculo.

Because pieza is a feminine noun in Spanish, so its plural article must also be feminine:

• la pieza = the piece
• las piezas = the pieces

In Spanish, articles have to agree with the noun in gender and number.

Because the subject is las piezas, which is plural.

The verb formar has to agree with the subject:

• la pieza forma = the piece forms
• las piezas forman = the pieces form

So forman is the 3rd person plural form of formar in the present tense.

It is the present indicative.

Here it can mean:

• the pieces form a circle
• the pieces are forming a circle in some contexts
• the pieces make up a circle

In a simple sentence like this, the present tense is the normal choice.

Because many pieces together make one circle.

So the sentence is saying:

• The pieces = plural subject
• form = plural verb
• a circle = singular result

This is exactly parallel to English: The pieces form a circle.

Because círculo is a masculine noun.

• un círculo = a circle
• el círculo = the circle

A lot of nouns ending in -o are masculine, and círculo is one of them.

The accent mark shows where the stress goes: CÍR-cu-lo.

Without the accent, Spanish stress rules would make you stress it differently. The written accent tells you clearly that the stress is on the first syllable.

In standard Spain Spanish:

• piezas is pronounced roughly PYEH-thas
• círculo is pronounced roughly THEER-koo-lo or SEER-koo-lo depending on the speaker and region

The important Spain feature is that z and c before e/i are often pronounced like the th in think:

• piezas → th
• círculo → the first c before i also has that sound

In much of Latin America, those sounds are usually pronounced with s instead.

Piezas can mean pieces, but the exact kind of pieces depends on context.

For example, it could refer to:

• puzzle pieces
• game pieces
• pieces of an object
• components or parts

So the sentence itself does not tell you exactly what kind of pieces they are; context does that.

Formar is very natural when separate things together create a shape, figure, group, or structure.

So:

• Las piezas forman un círculo = the pieces form a circle

You may also hear hacer in some situations, but formar sounds especially appropriate for the idea of making up a shape or arrangement.

Yes, it can be understood, and in some contexts native speakers might say it. But forman un círculo is often the more natural choice when you mean that the pieces are arranged so that their overall shape is a circle.

A useful contrast is:

• forman un círculo = they constitute / make up a circle
• hacen un círculo = they make a circle

Both can work, but forman sounds a bit more precise here.

Yes, but it means something slightly different.

• Las piezas forman un círculo = the pieces themselves create a circle shape
• Las piezas están en círculo = the pieces are arranged in a circular layout / in a circle

The difference is small, but forman focuses on the shape produced, while están en círculo focuses more on the position or arrangement.

Normally, no. You would usually say:

• forman un círculo

In Spanish, countable singular nouns usually need an article or determiner. Since círculo is a singular countable noun, un is the normal choice here.

Yes, Spanish word order is flexible, but the normal order here is:

• Las piezas forman un círculo.

You could sometimes move things around for emphasis, but it would sound less neutral. For a learner, the standard order is the best one to use.

Here, formar is being used as a normal, non-reflexive verb:

• Las piezas forman un círculo.

You might also see:

• Se forma un círculo. = A circle is formed.
• Las piezas se forman... = this would usually mean something different and is not the natural version here

So in this sentence, forman without se is the straightforward and correct form.

The dictionary form is formar.

That is the infinitive, meaning to form.

A few present-tense forms are:

• yo formo = I form
• tú formas = you form
• él/ella forma = he/she forms
• ellos/ellas forman = they form

So forman matches las piezas.