Tom sitter på stolen til venstre for Anna.

Stolen is the definite singular form of stol. In Norwegian, you indicate "the X" by adding a suffix to the noun:

• stol = a chair
• stolen = the chair
  Since Tom is sitting on a specific chair, you need the definite form.

The verb sitte ("to sit") typically pairs with på when referring to seating furniture:

• på stolen (on the chair)
• på en benk (on a bench)
  If you enter an enclosed space, like a car or a boat, you use i (e.g. i bilen, i båten).

Til venstre for is a set phrase meaning to the left of. Literally:

• til = to
• venstre = left
• for = of
  Together they locate something relative to another object: X til venstre for Y = X is to the left of Y.

Norwegian does not have a separate continuous aspect. The simple present sitter covers both:

• habitual actions: jeg sitter ofte her (I often sit here)
• ongoing actions: jeg sitter nå (I am sitting now)
  Context tells you which meaning applies.

You can move the entire location phrase around, but keeping på stolen til venstre for Anna together sounds most natural. Splitting it might cause momentary confusion:

• Preferred: Tom sitter på stolen til venstre for Anna
• Less natural: Tom sitter til venstre for Anna på stolen

In Bokmål, most nouns ending in a consonant are masculine and take -en in the definite singular. A dictionary will list stol as masculine, or you can practice common noun endings:

• Masculine → -en
• Feminine → -a or -en
• Neuter → -et