No aprietes el tapón tan fuerte.

Because this is a negative command addressed to tú.

In Spanish, affirmative and negative tú commands are formed differently:

• Aprieta = affirmative command: Tighten / Press
• No aprietes = negative command: Don’t tighten / Don’t press

Negative commands use the present subjunctive form:

• apretar → aprietes

So:

• Aprieta el tapón. = Tighten the cap.
• No aprietes el tapón. = Don’t tighten the cap.

No aprietas would usually mean you don’t tighten or you are not tightening, not a command.

Because apretar is a stem-changing verb.

In the present tense and in the present subjunctive, the e in the stem changes to ie in many forms:

• apretar
• yo aprieto
• tú aprietas
• él/ella aprieta

The present subjunctive is based on the yo form:

• yo aprieto → remove -o → apriet-
• add subjunctive endings:
  • apriete
  • aprietes
  • apriete
  • apretemos
  • apretéis
  • aprieten

That is why the sentence has no aprietes.

It can mean both, depending on context.

Apretar is a very common verb with several related meanings:

• to tighten: Apretar un tornillo = tighten a screw
• to squeeze: Apretar una naranja = squeeze an orange
• to press: Apretar un botón = press a button

In No aprietes el tapón tan fuerte, the most natural meaning is don’t tighten the cap so hard / so tightly.

Spanish often uses the definite article where English might use the, your, or sometimes nothing at all.

Here el tapón means the cap / the stopper. It refers to a specific cap already understood from the situation.

Spanish normally keeps the article with concrete objects:

• Cierra la puerta. = Close the door.
• Lávate las manos. = Wash your hands.
• No aprietes el tapón. = Don’t tighten the cap.

Leaving out the article here would sound incomplete or unnatural.

Tapón usually means a cap, stopper, or plug, depending on the object.

For example:

• bottle cap
• stopper on a container
• plug that closes an opening

In this sentence, tapón most likely means the cap of a bottle, jar, or similar container.

In Spain, tapón is very natural for this kind of object. The exact English translation depends on what kind of container you are talking about.

Because tan means so, while muy means very.

• tan fuerte = so hard / so tightly
• muy fuerte = very hard / very strongly

In this sentence, tan suggests degree in a way that often sounds like a warning or reaction:

• No aprietes el tapón tan fuerte. = Don’t tighten the cap so hard.

If you said No aprietes el tapón muy fuerte, it would still be understandable, but tan fuerte is more idiomatic in this kind of sentence.

Literally, fuerte often means strong, but in many contexts it is translated more naturally as:

• hard
• firmly
• tightly
• forcefully

So in this sentence, tan fuerte does not mean the cap itself is strong. It describes how someone is tightening it:

• too hard
• so tightly
• with too much force

A natural English translation is usually Don’t tighten the cap so tightly or Don’t tighten the cap so hard.

Yes, that is possible, but it means something slightly different in emphasis.

Compare:

• No aprietes el tapón tan fuerte.

  • Focuses on how strongly you tighten it.
  • Very natural for don’t tighten it so hard/tightly.

• No aprietes tanto el tapón.

  • Focuses more on the amount/degree of tightening.
  • Also natural: Don’t tighten the cap so much.

Both are correct, but the original sentence sounds especially natural when talking about physical force.

Yes.

• No aprietes el tapón tan fuerte. = Don’t tighten the cap so hard.
• No aprietes el tapón demasiado fuerte. = Don’t tighten the cap too hard.

The difference is:

• tan fuerte = so hard
• demasiado fuerte = too hard

Both are common. Demasiado makes the idea of excess more explicit.

Because Spanish often leaves out subject pronouns when they are clear from the verb form.

In no aprietes, the verb already shows that the speaker is talking to tú.

So:

• No aprietes el tapón tan fuerte.
• Tú no aprietes el tapón tan fuerte.

Both are possible, but the version without tú is more normal unless you want special emphasis, contrast, or clarification.

Yes. No aprietes is the tú form, which is informal singular you.

In Spain, you might also hear:

• No apriete el tapón tan fuerte. = formal singular (usted)
• No apretéis el tapón tan fuerte. = informal plural (vosotros), very common in Spain
• No aprieten el tapón tan fuerte. = formal plural (ustedes), or plural in many Latin American varieties

So the original sentence is talking to one person informally.

Grammatically, it is a negative command. In real life, though, the tone can range from:

• a direct instruction
• a warning
• advice
• a request

Spanish negative commands do not always sound harsh. Tone depends on voice, situation, and relationship.

For example:

• said sharply: a firm instruction
• said gently: friendly advice

So grammatically it is a command, but pragmatically it may simply be a helpful warning.