Bitte spüle den Teller ab.

Because this is an imperative sentence, meaning a command or request.

In German, when you give a command to du (one person you know well), the verb often comes first:

• Spüle den Teller ab. = Wash/rinse the plate.

Adding bitte makes it more polite:

• Bitte spüle den Teller ab.

The subject du is not said, but it is understood.

Spüle is the du-imperative form of abspülen.

The normal du form is:

• du spülst ab = you wash/rinse off

But in a command, German usually removes the -st ending:

• Spüle ab!

So:

• du spülst den Teller ab = you rinse the plate
• Spüle den Teller ab! = Rinse the plate!

Because abspülen is a separable verb.

The full infinitive is:

• abspülen

But in a normal main clause or command, the prefix ab- separates and moves to the end:

• Ich spüle den Teller ab.
• Bitte spüle den Teller ab.

This is very common in German with separable verbs.

spülen means to rinse or to wash with water.

abspülen often means to rinse off, to wash off, or in everyday context to do the washing up / wash dishes by hand, depending on the object.

With den Teller, abspülen suggests rinsing or washing the plate off. In many situations, English might simply translate it as wash the plate.

So the ab- adds the idea of washing something off or clean.

Because den Teller is in the accusative case, which is used for the direct object.

Here, the plate is the thing being washed, so it is the direct object.

• nominative: der Teller = the plate
• accusative: den Teller = the plate

Since Teller is masculine, der changes to den in the accusative.

Bitte means please here.

It makes the command sound more polite:

• Spüle den Teller ab. = Rinse the plate.
• Bitte spüle den Teller ab. = Please rinse the plate.

German allows bitte in different positions:

• Bitte spüle den Teller ab.
• Spüle bitte den Teller ab.

Both are natural.

It is usually neutral to polite, especially because of bitte.

Without bitte, the command can sound stronger:

• Spüle den Teller ab. = more direct

With bitte, it becomes softer:

• Bitte spüle den Teller ab.

Still, it is a command addressed to du, so it is something you would normally say to a child, friend, family member, or someone you address informally.

The sentence is addressed to one person in the informal form, so the understood subject is du.

German imperatives change depending on who you are talking to:

• du: Spüle den Teller ab.
• ihr: Spült den Teller ab.
• Sie: Spülen Sie den Teller ab.

So this sentence is specifically for one person you address as du.

Yes. In everyday German, both Spüle and Spül can occur as imperative forms for du.

• Spüle den Teller ab.
• Spül den Teller ab.

The form with -e can sound a bit more careful, standard, or slightly more formal/traditional in some contexts. The shorter form is very common in spoken German.

Both are correct.

Yes. German is flexible with bitte.

All of these are possible:

• Bitte spüle den Teller ab.
• Spüle bitte den Teller ab.
• Spüle den Teller bitte ab.

The most common natural choices are usually the first two. The meaning stays basically the same, though the rhythm and emphasis may change slightly.

If you turn it into a normal statement, you use the regular conjugated verb and include the subject:

• Du spülst den Teller ab. = You rinse the plate.
• Ich spüle den Teller ab. = I rinse the plate.

This helps show the pattern:

• infinitive: abspülen
• statement: Du spülst den Teller ab.
• command: Spüle den Teller ab.

So the command is closely related to the normal du form, but without du and usually without -st.