Jeg ser refleksjonen i vinduet.

In Norwegian Bokmål the definite article is not a separate word placed before the noun. Instead, you add a suffix to the noun itself.
• For masculine and feminine nouns you add -en (refleksjon → refleksjonen).
• For neuter nouns you add -et (vindu → vinduet).

You choose the suffix based on the noun’s gender:
• Masculine/feminine nouns take -en (e.g. bil → bilen, refleksjon → refleksjonen)
• Neuter nouns take -et (e.g. hus → huset, vindu → vinduet)

Norwegian verbs only inflect for tense, not for person or number. That means ser stays the same for jeg, du, han, vi, dere, de:
• jeg ser
• du ser
• han ser
…and so on.

• i = “in” or “inside,” so i vinduet means you see the reflection inside/through the glass.
• på = “on,” which would suggest you see something resting on the surface of the window (e.g. frost or dirt). To emphasize looking at the surface, you’d say på vinduet, but for a reflection you normally use i vinduet.

• ser = “see” (to perceive something visually, often unintentionally).
• ser på = “look at” (to direct your gaze purposely toward something).
In this sentence we’re simply perceiving the reflection, so we use ser.

Approximate pronunciation in IPA: /reˈfleksjuːnən/
You can break it down roughly as: reh-FLEK-syo-nen, with stress on the second syllable.

Yes. Basic word order is SVO:
• Subject: Jeg
• Verb: ser
• Object: refleksjonen
• Adverbial/Location: i vinduet