Χτύπησα το γόνατό μου στο τραπέζι.

Because Greek usually leaves out subject pronouns when the verb already makes the subject clear.

Χτύπησα means I hit / I bumped by itself, so εγώ is not necessary. You could add εγώ for emphasis, contrast, or clarification:

Εγώ χτύπησα το γόνατό μου στο τραπέζι.
= I hit my knee on the table.

Χτύπησα is the aorist form, which is the normal Greek past tense for a single completed action.

So here it means something like:

• I hit
• I bumped
• I knocked

It describes one event that happened and is over. If you wanted an ongoing or repeated past action, Greek would use a different form.

Yes. The dictionary form is usually given as χτυπάω or χτυπώ, both meaning to hit, strike, knock, beat, and in some contexts even to ring.

Its aorist past form is χτύπησα.

So:

• χτυπάω / χτυπώ = I hit / I am hitting
• χτύπησα = I hit / I bumped

In this sentence, it most naturally suggests accidentally banging your knee.

In Greek, when you use a possessive word like μου meaning my, you normally also keep the definite article.

So Greek says:

• το γόνατό μου = my knee
• το σπίτι μου = my house
• το βιβλίο μου = my book

This is very normal Greek structure. English does not use the here, but Greek usually does.

Here μου means my.

More technically, it is a short possessive clitic form. It is also the genitive form of the pronoun for me.

That is why μου can mean different things in different sentences:

• το βιβλίο μου = my book
• μου μίλησε = he/she spoke to me

In your sentence, it clearly shows possession: my knee.

This is an accent rule.

The basic noun is γόνατο. When a word like μου follows it, Greek spelling often adds an extra accent mark if needed. That is why you get:

• γόνατο
• το γόνατό μου

So this is not a different noun form you need to memorize separately. It is the same word, with an accent change caused by the following unstressed word μου.

Στο is a contraction of σε + το.

So:

• σε = in, at, on, to
• το = the
• στο = in the / at the / on the / to the

Greek uses this contraction very often, and it is completely standard.

Because Greek σε has a wider range of meanings than English to.

Depending on context, σε can mean:

• to
• in
• at
• on

With a verb like χτυπάω, στο τραπέζι often means contact with something, so in natural English it becomes on the table or against the table.

So Greek is using one preposition where English may choose different ones.

Both are in the accusative here.

• το γόνατό μου is the direct object of χτύπησα
• στο τραπέζι uses σε, which also takes the accusative

A useful thing to know is that many neuter nouns in Greek have the same form in the nominative and accusative singular, so they may look like dictionary forms.

Yes. Greek word order is more flexible than English word order.

The sentence you have is a neutral, natural way to say it:

Χτύπησα το γόνατό μου στο τραπέζι.

But Greek can move things around for emphasis, for example:

Στο τραπέζι χτύπησα το γόνατό μου.
This puts more focus on the table.

Even though the order can change, the article and possessive usually stay with the noun:

• το γόνατό μου

A rough guide is:

kh-TI-pi-sa

A few points:

• χ is not English ch as in chair
• it is more like the sound in Scottish loch or German Bach
• τύ is the stressed part
• υ here sounds like i

So the stress is on the second syllable:

χΤΥπησα

That same stress pattern is important in the full sentence too.