Participanții se încălzesc la linia de start.

Because Participanții means the participants, not just participants.

Romanian puts the definite article at the end of the noun:

• participant = participant
• participanți = participants
• participanții = the participants

The double i happens because:

• one i is part of the plural ending
• the other i is part of the definite article

So -ii here is normal.

Se is a reflexive marker. In this sentence, a se încălzi means to warm up.

Romanian often uses a reflexive form where English does not. So:

• a încălzi = to heat/warm something
• a se încălzi = to warm oneself up, to warm up

For example:

• Încălzesc supa. = I heat the soup.
• Mă încălzesc. = I am warming up.

So Participanții se încălzesc is literally something like The participants warm themselves up, but in natural English we just say The participants are warming up.

Încălzesc is present tense, 3rd person plural.

It matches the subject Participanții, which is plural.

So:

• eu încălzesc = I heat / I am heating
• ei/ele încălzesc = they heat / they are heating

With the reflexive se, se încălzesc means:

• they warm up
• or they are warming up

Romanian present tense often covers both the simple present and the present progressive in English.

Because Romanian does not usually need a separate helper verb like English am/is/are for this kind of action.

The simple present in Romanian can express:

• a general action: They warm up
• an action happening now: They are warming up

Context tells you which one is meant. In a sports scene like this, the natural English translation is usually are warming up.

Because linia is the definite form of linie.

• linie = line
• linia = the line

Again, Romanian puts the definite article at the end of the noun.

So:

• linia de start = the starting line / literally the line of start

De start means something like of start or for starting, and together linia de start means starting line.

This is a very normal Romanian noun pattern:

• X de Y

In sports Romanian, start is commonly used as a loanword from English. So linia de start is very natural.

You may also see more native-looking alternatives in other contexts, but start is extremely common in sports language.

Here la means at.

So:

• la linia de start = at the starting line

Romanian often uses la for being at a place, point, or event location.

An English speaker might expect pe because the participants are physically on the line, and Romanian can sometimes use pe linia de start too. But la linia de start is very natural when the idea is simply at the starting line/start area.

A few important ones:

• ț = ts, like the end of cats
• ă = a short neutral vowel, like the a in about
• î = a Romanian vowel with no exact English equivalent; it sounds like a tight central uh/i sound

So roughly:

• Participanții ≈ par-ti-ci-PAN-tsii
• încălzesc ≈ uhn-cuhl-ZESK
• linia ≈ LEE-nee-a

These are only rough English approximations, but they can help you get started.

Yes. Romanian often omits the subject when it is clear from the verb form or from context.

Because încălzesc is 3rd person plural, Se încălzesc la linia de start can mean:

• They are warming up at the starting line

Including Participanții makes the sentence more explicit:

• The participants are warming up at the starting line

So both are possible; the full version is just clearer and more specific.