Really is an adverb because it qualifies verbs, adjectives and other adverbs.

A set smile or expression does not change, and often hides what someone is really thinking. Shamefaced adjective. With an expression that shows you feel ashamed about something. Slack-jawed adjective. Informal with your mouth open. Used about the expression on people’s faces. Showing that you think something. The adverbial use of 'real' is really annoying to me. So I don't think it's appropriate to use it, ever. That doesn't prevent people from doing so. – Catija ♦ Aug 10 '16 at 21:27 In my family, 'real' as an adverb is.

Infact, one of the key identification of adverbs is that a lot of them end in -ly.

5 Adjective Words

Let us look at some of the usage examples of the adverb ‘really’.

1. Definition of REALLY (adverb, interjection): very; used for emphasis; completely; used for saying what is true; used for showing surprise.
2. Definition of really. 1 a: in reality: actually things as they really are there was nothing peculiar about her doing this, really — Peter Taylor. B: truly, unquestionably —used as an intensifier a really beautiful day. C: very sense 1 look really close he runs really.
3. We use the adverb really when we want to emphasise something: That pasta was really delicious. I really like those photographs. Can we see them on the computer? Emigrating to Canada was a really big decision for the whole family. We also use really to question whether what someone says is true: They don’t look pleased to me. Are they really.

Really Adjective Or Adverb

i) Qualifying a verb:
a) He really stays up all night. (qualifies verb ‘stays’)
b) Did Sudhir really say that? (qualifies verb ‘say’)
c) I can never really forget that day. (Qualifies verb forget).
d) You don’t really learn from your mistakes. (qualifies the verb learn).

Now, let us look at examples where the adverb ‘really’ is qualifying adjectives:

Ex1: He is really fat (qualifies adjective ‘fat’).
Ex2: It was really scary in the jungle (qualifies adjective ‘scary’)
Ex3: This place is really beautiful (qualifies adjective beautiful).
Ex4: Bullet train journey is really amazing (adjective amazing)

And then, the third set of usage where ‘really’ is sued to modify other adverbs:

100 Descriptive Adjectives

Really Adjectives

i) Sudhir runs really fast.
ii) Mahesh works really hard.
iii) I didn’t really work hard enough.
iv) Why don’t they never really tire of complaining?