La realidad es diferente.

In Spanish, abstract nouns are often used with the definite article when they are the subject of a sentence. So la realidad means reality in a general sense.

That is why La realidad es diferente sounds natural in Spanish, even though English usually drops the in this kind of sentence.

Realidad is a feminine noun, so it takes la.

A useful pattern is that many nouns ending in -dad are feminine:

• la ciudad
• la verdad
• la posibilidad
• la realidad

So even though it does not end in -a, it is still feminine.

It uses es because ser is normally used for descriptions seen as general, essential, or defining characteristics.

Here, diferente describes reality as a general statement, so ser is the natural choice:

• La realidad es diferente.

Using estar would suggest a temporary state or change, which does not fit as well here.

Es is the third person singular present form of ser.

So it means:

• he/she/it is
• you are (formal singular)

In this sentence, it matches la realidad, which is singular:

• La realidad es diferente. = Reality is different.

Some Spanish adjectives have the same form for masculine and feminine singular. Diferente is one of them.

So you get:

• el libro es diferente
• la realidad es diferente

The adjective only changes in the plural:

• los libros son diferentes
• las realidades son diferentes

Yes. Distinta is a very common alternative to diferente and works well here.

So both are natural:

• La realidad es diferente.
• La realidad es distinta.

They are very close in meaning. In many contexts, they are interchangeable.

Yes, but La realidad es diferente is the normal, neutral order.

You may also hear:

• Es diferente la realidad

That version sounds more marked or emphatic, and it is less neutral. For a learner, the standard word order is the best one to use first:

• subject + verb + adjective

A simple pronunciation guide:

• la → lah
• realidad → reh-ah-lee-DAD
• es → ess
• diferente → dee-feh-REN-teh

The stress falls on:

• realiDAD
• difeRENte

In normal speech, es is short and unstressed.

Here is the breakdown:

• La = definite article
• realidad = noun
• es = verb (ser, present tense, third person singular)
• diferente = adjective

So the structure is:

article + noun + verb + adjective

Often, yes, if you say what something is different from:

• La realidad es diferente de lo que imaginamos.
• Esta situación es diferente de la anterior.

But in your sentence, the comparison is left unstated because it is understood from context:

• La realidad es diferente.

That is completely correct and natural.

No, that would not sound natural in standard Spanish.

You need:

• La realidad es diferente.

Leaving out the article here would sound incomplete or ungrammatical in normal usage.