Circulus in tabula ducitur.

Because ducitur is passive: is drawn.

In a passive sentence, the thing receiving the action becomes the subject. So:

• circulus = a circle / the circle in the nominative singular
• ducitur = is drawn

If the sentence were active, you would expect circulum as the direct object:

• Puer circulum in tabula ducit = The boy draws a circle on the tablet/board

But in the passive version:

• Circulus in tabula ducitur = A circle is drawn on the tablet/board

So circulus is nominative because it is now the grammatical subject of the passive verb.

Ducitur is:

• 3rd person singular
• present tense
• passive voice
• from the verb ducere

So it literally means is led in the most basic sense of ducere, but in this context it means is drawn or is traced.

The ending -tur is a very common sign of the passive in Latin.

For comparison:

• ducit = he/she/it draws or leads
• ducitur = he/she/it is drawn or is led

Yes—ducere very often means to lead. But like many Latin verbs, it has a wider range of meanings.

Its basic idea is something like to pull/lead along. From that, it can also mean:

• lead
• draw
• trace
• drag
• extend

So when talking about lines or shapes, Latin can use ducere for drawing them:

• lineam ducere = to draw a line
• circulum ducere = to draw a circle

This is a very natural Latin usage, even if it sounds a little unusual at first to an English speaker.

Because in takes different cases depending on the meaning.

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here, tabula is in the ablative singular, so in tabula means on the tablet/board or in the picture, depending on context.

The sentence describes where the circle is being drawn, not motion into something, so Latin uses in + ablative.

Compare:

• circulus in tabula ducitur = a circle is drawn on the tablet/board
• aliquid in tabulam mittitur = something is sent into/onto the tablet/board
  (here the idea would be motion toward it)

Tabula is ablative singular.

It is ablative because it follows in in the sense of location: on the tablet/board, in the picture, and so on.

This is a very common pattern in Latin:

• in urbe = in the city
• in aqua = in the water
• in mensa = on the table
• in tabula = on the tablet/board

So the ablative here is not random—it is required by the preposition in when in means location.

Tabula can mean several things in Latin, depending on context, such as:

• board
• tablet
• writing tablet
• plank
• picture/panel

So in tabula could mean different things in different situations:

• on the tablet
• on the board
• in the picture

If the meaning has already been supplied to the learner, that will tell you which sense is intended here. Grammatically, though, the form is the same.

Because Latin has no articles.

So circulus can mean:

• a circle
• the circle

And tabula can mean:

• a board
• the board
• a tablet
• the tablet

English has to choose a/an or the, but Latin usually leaves that to context.

So Circulus in tabula ducitur could be translated as:

• A circle is drawn on the board
• The circle is drawn on the board

The correct choice depends on context.

Yes. Ducitur is present tense, so the sentence means:

• A circle is drawn on the board
• or more naturally in some contexts, A circle is being drawn on the board

Latin present passive can often be translated either as a simple present passive or as an English progressive passive, depending on context.

So both ideas may fit:

• is drawn
• is being drawn

Yes. Latin word order is much freer than English word order because the endings show the grammatical roles.

So these can all express basically the same idea:

• Circulus in tabula ducitur
• In tabula circulus ducitur
• Ducitur circulus in tabula

However, the word order can affect emphasis.

For example:

• Circulus in tabula ducitur may feel neutral.
• In tabula circulus ducitur puts more focus on where it is drawn.
• Ducitur circulus in tabula puts the verb first, which can feel more vivid or stylistic.

English cannot move words around so freely without sounding strange, but Latin often can.

An active version would add someone doing the action, for example:

• Magister circulum in tabula ducit = The teacher draws a circle on the board
• Puer circulum in tabula ducit = The boy draws a circle on the board

Notice the difference:

In the active:

• circulum is accusative, the direct object

In the passive:

• circulus is nominative, the subject

So:

• Circulum in tabula ducit = He/she draws a circle on the board
• Circulus in tabula ducitur = A circle is drawn on the board

In Latin, in with the ablative can cover both in and on, depending on the noun and the situation.

So although the Latin says in tabula, good English will often say:

• on the board
• on the tablet

rather than in the board.

This is normal in translation. Latin and English do not always divide spatial ideas in exactly the same way. So the translator should choose the most natural English expression, not translate each word mechanically.