Learn Finnish: Menen kotiin taksilla, koska sataa. - I go home by taxi because it is raining.

The ending -lla is the Adessive case. While it often means 'on top of' or 'at', it is also used to express the means of transportation you use to get somewhere. So taksilla means 'by taxi'. You would use the same ending for other vehicles, like junalla (by train) or bussilla (by bus).

Why is the word for home kotiin instead of just koti?

Because you are expressing movement towards a destination. Finnish uses the Illative case for this. Koti means 'home' as a stationary place, but when you are going to your home, the target direction makes it kotiin.

Where is the word for 'it' in the phrase koska sataa?

Finnish does not use dummy subjects like the 'it' in English for weather conditions. The verb sataa (to rain) simply stands alone in the third-person singular to mean 'it is raining'.

Can I change the word order to say Menen taksilla kotiin?

Yes, you can! Finnish word order is flexible. Saying Menen kotiin taksilla slightly emphasizes how you are getting there, whereas Menen taksilla kotiin places slightly more focus on your destination. Both are perfectly natural.