Learn Russian: Слева от окна большое кресло. - To the left of the window there is a big armchair.

Слева means "on the left" (location), answering the question где? (where?). Налево means "to the left" (direction of motion), answering the question куда? (where to?). Since the armchair is sitting still, we use the location word слева. When combined with от to mean "to the left of" something, it forms слева от.

Why does окно change to окна here?

The preposition от (from/of) always requires the Genitive case. Like many spatial relationship prepositions you are learning (such as вокруг and вдоль), the compound preposition слева от triggers the Genitive case. So, the neuter noun окно becomes окна.

The English translation says "there is", so why isn't есть used in the Russian sentence?

In Russian, when you are describing a scene or stating where a specific thing is located, the verb есть is usually dropped. You only use есть when you need to emphasize the actual existence or availability of something. Here, we are just painting a picture of the room's layout, so the verb is simply understood from the context.

Why is the adjective большое rather than большой or большая?