Bitte schneide die Zitrone vorsichtig, sonst verletzt du dich.

Please cut the lemon carefully, otherwise you'll hurt yourself.

Breakdown of Bitte schneide die Zitrone vorsichtig, sonst verletzt du dich.

• du: you
• bitte: please
• sonst: otherwise
• schneiden: to cut
• die Zitrone: the lemon
• die: the; (feminine, accusative)
• dich: yourself
• vorsichtig: careful / carefully
• sich verletzen: to hurt oneself / to injure oneself

Questions & Answers about Bitte schneide die Zitrone vorsichtig, sonst verletzt du dich.

Why does Bitte come first—does it always start a sentence?

Bitte often appears at the start of a polite request, like Please in English: Bitte schneide ....
It doesn’t have to come first, though. You can also say Schneide bitte die Zitrone vorsichtig ... or Schneide die Zitrone bitte vorsichtig .... The meaning stays basically the same; the placement mainly affects emphasis and flow.

Why is it schneide and not schneidest or schneiden?

Schneide is the imperative form for du (informal singular):

• infinitive: schneiden (to cut)
• present tense: du schneidest (you cut/you are cutting)
• imperative: (du) schneide! (cut!)
  In the du-imperative, German usually drops the -st ending and often keeps -e: schneid(e)! Both schneide and schneid are possible; schneide can sound a bit more careful/polite.

Why is the pronoun du included? Don’t imperatives usually omit it?

Yes—normally you omit it: Schneide die Zitrone ... is perfectly standard.
Including du (..., sonst verletzt du dich) is not part of the imperative; it belongs to the second clause (otherwise you’ll hurt yourself). In the first clause, du is omitted (that’s normal).

What’s the difference between schneiden and abschneiden here?

schneiden is a general to cut. abschneiden is more like to cut off (separating something from something else).
With a lemon, schneiden works well if you mean cutting it (into slices, halves, etc.). If you specifically meant cutting off a piece (e.g., a lemon peel strip), abschneiden could fit: Bitte schneide ein Stück Zitrone ab.

Why is it die Zitrone—what does die tell me?

die is the definite article (the) and it also shows:

• gender: Zitrone is feminine (die Zitrone)
• case: here it’s accusative (direct object), but die looks the same in nominative and accusative for feminine nouns, so you don’t see a change:
  
  • nominative: die Zitrone ist...
  • accusative: ich schneide die Zitrone.

Where does vorsichtig go, and could it be placed somewhere else?

vorsichtig is an adverb meaning carefully. In German it often comes after the object in simple commands: Schneide die Zitrone vorsichtig.
You can move it for emphasis:

• Bitte schneide vorsichtig die Zitrone ... (puts focus on carefully)
  Most neutral is the original: object first, then adverb.

What does sonst mean exactly, and is it the same as oder?

sonst means otherwise / or else and introduces a consequence if the first part isn’t followed: ..., sonst ...
It’s not the same as oder (or). oder offers alternatives; sonst warns about a result:

• Mach das, sonst passiert X. = Do that, otherwise X happens.

Why is there a comma before sonst?

Because this is a sentence with two clauses:
1) Bitte schneide die Zitrone vorsichtig (main clause / command)
2) sonst verletzt du dich (another clause giving the consequence)
German normally uses a comma to separate clauses like this, especially when one clause is introduced by a connector such as sonst.

Why is it verletzt du dich and not du verletzt dich? What’s the word order rule?

After sonst, German often uses verb-second (V2) word order, meaning the finite verb comes in the second position. If sonst takes the first position, the verb comes next:

• sonst (position 1) + verletzt (position 2) + du
  • dich
    So sonst verletzt du dich is the normal structure.

What tense is verletzt here—present or future?

It’s present tense, but German often uses present to talk about the near future, especially in warnings and conditions. Here it means: otherwise you’ll hurt yourself.

Why do we need dich—what kind of pronoun is it?

dich is the accusative reflexive pronoun for du. The verb sich verletzen means to hurt oneself (or to get hurt).

• ich verletze mich
• du verletzt dich
• er verletzt sich
  So dich shows the action comes back to the subject (you hurt yourself).

Is sich verletzen the same as sich schneiden? Would Germans say sonst schneidest du dich?

Both are possible but slightly different:

• sich schneiden = to cut yourself (often with a knife, paper, etc.)
• sich verletzen = to injure yourself (more general)
  With cutting a lemon, sonst schneidest du dich is very natural and specific. sonst verletzt du dich is a broader warning.

How would this change in a formal/polite Sie version?

You’d use the Sie-imperative, which keeps the verb and includes Sie:
Bitte schneiden Sie die Zitrone vorsichtig, sonst verletzen Sie sich.
(Or more specifically: ..., sonst schneiden Sie sich.)