No seas tonto.

Because No seas tonto uses a negative command addressed to tú. In Spanish, negative tú commands are formed with the present subjunctive, not the present indicative.

So the verb ser goes like this in the present subjunctive:

• yo sea
• tú seas
• él/ella/usted sea
• nosotros seamos
• vosotros seáis
• ellos/ellas/ustedes sean

That is why Spanish says No seas..., not No eres....

Compare:

• Eres tonto. = a statement
• No seas tonto. = telling someone not to act that way

Yes. It is a negative informal command directed at one person: tú.

A useful rule is:

• affirmative tú command: usually based on the 3rd person singular of the present indicative
• negative tú command: use no + present subjunctive

Examples:

• Ven. = come
• No vengas. = don’t come

So here:

• Sé bueno. = be good
• No seas tonto. = don’t be silly / don’t be stupid

Note that sé and seas both come from ser, but they are used in different command patterns.

Because Spanish usually omits subject pronouns when they are not needed. The verb form already shows who the subject is.

In No seas tonto, the form seas tells you the subject is tú, so saying tú is normally unnecessary.

You could say:

• Tú no seas tonto

but that sounds more emphatic, as if you are especially pointing at that person or contrasting them with someone else.

Because adjectives in Spanish usually agree with the person being described.

• tonto = used for a masculine person
• tonta = used for a feminine person

So:

• to a man or boy: No seas tonto
• to a woman or girl: No seas tonta

If you are speaking to a group, the form changes too:

• No seáis tontos = to a group of males or a mixed group in Spain
• No seáis tontas = to a group of females

Here it is functioning as an adjective after the verb ser.

The structure is:

• No seas
  • adjective

So tonto describes the subject: you.

Spanish often uses adjectives after ser in this way:

• Es amable.
• No seas cruel.
• No seas impaciente.

In some contexts tonto can also be used more like a noun when talking about a fool / an idiot, but in this sentence it is best understood as an adjective meaning someone is being foolish or silly.

It depends a lot on tone, context, and your relationship with the person.

In Spain, tonto can be:

• light and playful: like don’t be silly
• annoyed or insulting: closer to don’t be stupid

So No seas tonto is not automatically extremely harsh, but it can definitely sound rude if said sharply.

For a softer tone, Spanish speakers might say things like:

• No seas bobo
• No digas tonterías
• No seas así
• No seas tonto, hombre (this can sound more conversational depending on tone)

So the sentence itself is common, but the tone matters a lot.

They are grammatically and pragmatically different.

• No seas tonto = a command or warning: don’t be silly / don’t act stupid
• No eres tonto = a statement: you’re not stupid

So:

• No seas tonto tells someone how not to behave
• No eres tonto comments on what the person is like

This is an important distinction because Spanish often uses ser + adjective both for description and in commands, but the verb form changes the meaning completely.

The sentence you have is informal singular: tú.

Other versions are:

• No sea tonto / tonta = formal singular (usted)
• No seáis tontos / tontas = informal plural in Spain (vosotros/vosotras)
• No sean tontos / tontas = formal plural (ustedes)

For Spain specifically:

• with friends, children, or people you know well: No seas...
• with one person you address as usted: No sea...
• with several people informally: No seáis...

Yes. No seas tan tonto is very natural.

Adding tan means so:

• No seas tonto = don’t be silly / don’t be stupid
• No seas tan tonto = don’t be so silly / don’t be so stupid

The version with tan often sounds a bit more expressive because it emphasizes the degree.

You may also hear:

• No seas tan tonta
• No seáis tan tontos

A simple pronunciation guide is:

• No ≈ noh
• seas ≈ SEH-as
• tonto ≈ TON-toh

A few useful notes for Spain:

• s is a clear s sound
• t is a clean dental t, softer than a very strong English t
• the stress is on SE- in seas
• the stress is on TON- in tonto

A rough full pronunciation is:

NO SEH-as TON-toh

In natural speech, it may sound quite fluid, almost like: No seastonto, but the words are still separate.