Learn Finnish: Minä katson taivaalle kaukoputkella. - I look at the sky with a telescope.

The verb katsoa (to look) requires you to express where your gaze is directed rather than taking a basic object. You usually look into something (illative case) or onto something (allative case, -lle). For the sky, Finns commonly use the -lle ending, so katson taivaalle essentially means "I direct my gaze onto the sky."

The basic word for "sky" is taivas. Why does it change to taivaa- before the ending?

Taivas belongs to a specific group of Finnish nouns ending in -as or -äs. When adding an ending like -lle, the final -s drops out and the preceding vowel doubles. That gives us the stem taivaa-, making the final word taivaalle.

How does the ending on kaukoputkella translate to "with"?

In Finnish, you don't use a separate word for "with" when talking about a tool or instrument you use to do something. Instead, you use the adessive case ending -lla or -llä (which normally means "on"). Attached to kaukoputki (telescope), the ending in kaukoputkella means "by means of a telescope" or "with a telescope."

What are the parts of the compound word kaukoputki?

It is made up of kauko- (a prefix meaning "far" or "distant") and putki ("tube" or "pipe"). So, a telescope in Finnish is quite literally a "far-tube".