No eches la pelota tan alto.

Because this is a negative command addressed to tú.

In Spanish, negative commands for tú use the present subjunctive, not the normal present tense.

• Present tense: tú echas = you throw
• Negative command: no eches = don’t throw

So No eches la pelota tan alto means Don’t throw the ball so high.

A very common way to form the tú negative command is:

1. Start with the yo form of the present tense
   
   • yo echo
2. Remove the final -o
3. Add the opposite ending
   
   • for -ar verbs, use -es

So:

• echar
• yo echo
• eches

That gives:

• no eches

That is how Spanish makes a negative command.

• Echa la pelota = Throw the ball
• No eches la pelota = Don’t throw the ball

In Spanish, no normally goes directly before the verb in negative commands.

Yes. The subject pronoun tú is omitted because Spanish often leaves subject pronouns unstated when the verb form already makes the subject clear.

So this sentence could be expanded to:

• Tú no eches la pelota tan alto

But that sounds less natural unless you want emphasis, contrast, or clarity. Normally, Spanish just says:

• No eches la pelota tan alto

Spanish often uses the definite article more naturally than English does.

Here, la pelota means the ball, and it can refer to:

• a specific ball already understood in the situation, or
• simply the ball involved in the activity

In English, we might sometimes say Don’t throw the ball so high, and Spanish naturally uses la here.

Tan means so, while muy means very.

So:

• tan alto = so high
• muy alto = very high

In this sentence, tan alto fits the idea of Don’t throw it so high, which often suggests to that degree or that much.

If you said No eches la pelota muy alto, it would mean something like Don’t throw the ball very high. That is possible, but tan alto is more natural for the meaning given.

Because alto here is not describing the ball itself. It is describing how the ball is thrown — in other words, it works like an adverbial expression meaning high.

So:

• la pelota alta = the high ball / the tall ball-like object, where alta describes pelota
• echar la pelota alto = to throw the ball high, where alto describes the action

In this kind of use, Spanish often uses the masculine singular form as an adverb-like form:

• hablar claro = to speak clearly
• volar bajo = to fly low
• tirar alto = to throw high

So alto does not agree with pelota here.

Yes, echar can mean to throw, but Spanish also commonly uses other verbs depending on context.

Some common options are:

• echar = throw, toss, cast
• tirar = throw
• lanzar = throw, launch

So you may also hear:

• No tires la pelota tan alto
• No lances la pelota tan alto

All are possible, but echar is very common and natural in many everyday contexts.

It is informal singular, because it is a command to tú.

Here are the equivalent forms:

• tú: No eches la pelota tan alto
• usted: No eche la pelota tan alto
• vosotros: No echéis la pelota tan alto
• ustedes: No echen la pelota tan alto

Since you asked about Spanish from Spain, it is especially useful to notice the vosotros form:

• No echéis la pelota tan alto

Spanish can either use the full noun or replace it with a pronoun.

So both are possible:

• No eches la pelota tan alto = Don’t throw the ball so high
• No la eches tan alto = Don’t throw it so high

The version with la pelota names the object explicitly. The version with la is used when it is already clear what object you are talking about.

Eches is stressed on the first syllable:

• E-ches

A rough English approximation is EH-ches, with ch as in chair.

Also:

• e is like a short pure eh sound
• ch is the same as English ch
• the final es is clearly pronounced

So it sounds approximately like:

• EH-ches

In normal use, it is clearly a negative command: Don’t throw the ball so high.

However, Spanish negative subjunctive forms like no eches can also appear in clauses with other meanings in different sentence types. But in this sentence, standing alone, the natural reading is definitely a command or instruction.

So for a learner, the important point is:

• No eches ... = Don’t ...

Yes, grammatically that is the positive tú command:

• Echa la pelota tan alto

But there is an important pattern to notice:

• positive tú command: echa
• negative tú command: no eches

This is a very common contrast in Spanish:

• Habla / No hables
• Come / No comas
• Escribe / No escribas

So here:

• Echa = Throw
• No eches = Don’t throw