उल्टी करना एक लक्षण है।

In Hindi, many actions are expressed with a noun + करना pattern.

• उल्टी by itself commonly means vomit or vomiting
• उल्टी करना literally means to do vomiting, which is the natural Hindi way to say to vomit

So the sentence uses the full verbal expression उल्टी करना to talk about the act of vomiting.

Yes, but it depends on what you want to say.

• मुझे उल्टी है। = I am vomiting / I feel vomit-like / I have vomiting
• उल्टी करना = to vomit

So:

• उल्टी can function as a noun
• उल्टी करना functions as a verb phrase

In this sentence, the speaker is treating vomiting as an action or condition and labeling it as a symptom, so उल्टी करना works well.

एक literally means one, but it is also often used like a/an in English.

So:

• एक लक्षण = a symptom

Hindi does not always need an equivalent of a/an, but using एक is very common when introducing something as one item in a category. Here it helps mean vomiting is one symptom or vomiting is a symptom.

लक्षण means symptom or sign.

In medical contexts, it usually means symptom.
So एक लक्षण means a symptom.

A learner should also know that Hindi sometimes distinguishes:

• लक्षण = symptom/sign
• निशानी = sign/mark/indication, often more general

But in health-related language, लक्षण is the standard word.

है means is.

The sentence structure is:

• उल्टी करना = vomiting / to vomit
• एक लक्षण = a symptom
• है = is

So the full structure is:

• उल्टी करना एक लक्षण है।
• Vomiting is a symptom.

Hindi often places the verb at the end of the sentence, unlike English.

Hindi usually follows Subject–Object/Complement–Verb order, while English usually follows Subject–Verb–Complement.

So in English:

• Vomiting
  • is
    • a symptom

In Hindi:

• उल्टी करना
  • एक लक्षण
    • है

This is completely normal Hindi word order. The verb है comes last.

Here, उल्टी करना is being treated as a single idea: the act of vomiting.

Because it is a single action/concept, Hindi uses singular है:

• उल्टी करना एक लक्षण है। = Vomiting is a symptom.

If the subject were plural, then हैं would be used. But here the phrase refers to one general action, so है is correct.

Yes, करना is the infinitive form of करना = to do.

In उल्टी करना, the infinitive helps create a verbal noun or action phrase:

• करना = to do
• उल्टी करना = to vomit / vomiting

In sentences like this, the infinitive phrase can behave a bit like an English -ing form:

• Smoking is harmful.
• धूम्रपान करना हानिकारक है।

So उल्टी करना here is functioning as the subject of the sentence.

Yes, उल्टी एक लक्षण है is also possible, and it would sound very natural in many contexts.

There is a slight difference in feel:

• उल्टी एक लक्षण है = vomiting/vomit is a symptom
• उल्टी करना एक लक्षण है = the act of vomiting is a symptom

The version with करना focuses a little more on the action. The version without it is often more compact and common in medical-style statements too.

A helpful rough transliteration is:

ulṭī karnā ek lakṣaṇ hai

Very roughly for an English speaker:

• उल्टी = ul-tee with a retroflex ṭ sound
• करना = kur-naa
• एक = ek
• लक्षण = luk-shun or luk-shan, with a retroflex ṣ/ṇ quality in careful pronunciation
• है = hai like heh or hae, depending on accent

A more natural flowing pronunciation is something like:

ulṭī karnā ek lakṣaṇ hai

Yes, उल्टी is feminine as a noun.

However, in this sentence, agreement does not really show up in an obvious way because:

• करना stays in its basic infinitive form
• है does not change for gender

So even though उल्टी is feminine, the sentence remains:

• उल्टी करना एक लक्षण है।

Gender matters more in sentences where adjectives or verbs show agreement more clearly.

Yes, it is understandable and natural, especially in a medical or explanatory context.

That said, depending on context, native speakers might also say:

• उल्टी एक लक्षण है।
• उल्टी होना एक लक्षण है।

The version with होना can sometimes sound more natural when talking about symptoms appearing or happening:

• उल्टी होना = to have vomiting / for vomiting to occur

But उल्टी करना एक लक्षण है is still perfectly valid and clear.