Incido una linea nella lastra.

Incido is the first-person singular present indicative of incidere.

So:

• incidere = the infinitive, to engrave / to incise / to carve
• incido = I engrave / I incise / I carve

That means the sentence is talking about what I do:
Incido una linea nella lastra = I incise a line into the slab/plate.

Yes. Io is grammatically possible, but it is usually left out in Italian because the verb ending already shows the subject.

• incido already means I incise
• so io is often unnecessary

You would add io only for emphasis, contrast, or clarity, for example:

• Io incido una linea, tu ne incidi due.
  I incise one line; you incise two.

So the version without io is the most neutral and natural.

Here incidere means something like:

• to incise
• to engrave
• to cut into a surface

It suggests making a mark by cutting into the material, not just drawing on it with a pen or pencil.

So in this sentence, the line is being made in the material itself.

That is why incidere fits well with lastra: a slab, plate, or sheet of solid material.

Because Italian normally needs an article before a singular countable noun.

• una linea = a line
• linea by itself would usually sound incomplete in a normal sentence

So:

• Incido una linea... = I incise a line...

This works just like English in this case: a line is an indefinite singular object.

Because nella is the normal contraction of:

• in + la = nella

So:

• nella lastra = in the slab / into the slab / on the slab, depending on context

Italian commonly combines many prepositions with definite articles:

• nel = in + il
• nello = in + lo
• nella = in + la
• nei = in + i
• negli = in + gli
• nelle = in + le

So in la lastra would be wrong in standard Italian.

Because the verb incidere already suggests cutting into a surface.

Literally, nella often means in the, but in this context English naturally prefers into the because the action is penetrative: the line is being cut into the material.

So the Italian wording is normal, even though the best English translation may shift a little:

• nella lastra
  literally: in the slab
• natural English with this verb: into the slab

This is a good example of how you should translate the whole expression, not each word separately.

Because Italian uses definite articles very regularly.

• la lastra = the slab / the plate

In this sentence, nella lastra implies a specific slab or plate, not just any slab in general.

Italian often uses articles in places where English may or may not, but here both languages are fairly similar:

• nella lastra = in/into the slab

Lastra usually means a slab, plate, or sheet of some solid material.

The exact English word depends on context:

• stone: slab
• metal: plate or sheet
• glass: pane or sheet
• medical context: older use for an X-ray plate

In your sentence, it is some kind of flat solid surface that can be engraved or incised.

Linea is a very natural choice here because it means a line in a general geometric or visual sense.

Riga can also mean line, but it is often used for:

• a line of writing or text
• a ruled line
• a stripe or streak

With incidere, linea sounds more natural for a carved or engraved line in a surface.

So:

• incidere una linea = very natural
• incidere una riga = possible in some contexts, but less neutral here

The normal, neutral order is:

• Incido = verb
• una linea = direct object
• nella lastra = prepositional phrase

So the standard order is:

Incido una linea nella lastra.

Italian word order is more flexible than English, though. You can move parts around for emphasis:

• Nella lastra incido una linea.
  Emphasis on where the action happens.
• Una linea incido nella lastra.
  Less neutral; more marked or literary.

For everyday use, the original order is the best one.

Not necessarily. The present tense in Italian, like in English, can be used in different ways depending on context.

Incido can mean:

• I am incising a line into the slab
• I incise a line into the slab
• I do incise a line into the slab

It may describe:

1. an action happening now
2. a habitual action
3. a step in a procedure or demonstration

For example, in instructions or demonstrations, Italian often uses the present:

• Prendo lo strumento, poi incido una linea nella lastra.
  I take the tool, then I incise a line into the slab.

Linea is pronounced with three syllables:

li-ne-a

The stress is on li:

LÌ-ne-a

English speakers sometimes want to compress it, but in Italian the vowels are kept separate.

So it is not like a one-beat English line plus ending; it is clearly:

• li
• ne
• a

It is pronounced roughly:

in-CHEE-do

More accurately in Italian:

• in
• ci = like chee
• do

The stress is on ci:

in-CI-do

Important point: before i, Italian c is a soft sound, like English ch.

So ci sounds like chee, not like kee.

Sometimes, depending on the exact nuance, but nella is very natural here.

For example:

• sulla lastra = on the slab
• nella lastra = in/into the slab

With incidere, nella lastra works well because the mark is cut into the material, not just placed on top of it.

If you said sulla lastra, it might sound more like the action is done on the surface in a looser sense.
With engraving or incision, nella is often a better fit.

The direct object is una linea.

That is the thing being produced or made by the action of the verb:

• Incido what?
  una linea

The phrase nella lastra is not the direct object. It tells you where the line is incised.

So the structure is:

• incido = verb
• una linea = direct object
• nella lastra = location/resulting surface