Learn Swedish: Vi såg ett nytt ljus på himlen igår. - We saw a new light in the sky yesterday.

The word for light, ett ljus, is an ett-word (neuter). Normally, you add a single '-t' to an adjective for ett-words. However, the basic adjective is ny, which ends in a stressed vowel. When an adjective ends in a stressed vowel, you must add a double '-tt' for the neuter form, making it nytt.

Why do we say på himlen when the English translation says 'in the sky'?

Prepositions rarely match up perfectly between languages. In Swedish, the sky is often treated like a surface, so you use på (on) rather than i (in). If you were to say i himlen, it would usually mean 'in heaven' in a spiritual or religious sense.

What is the base form of himlen, and why does the spelling change?

The base word for sky is en himmel. When adding the definite suffix -en to mean 'the sky', the unstressed 'e' in the base word drops out. So himmel becomes himlen, which is much smoother to pronounce than 'himmelen'.

If I want to emphasize that this happened 'yesterday', can I start the sentence with igår?

Yes, you can move igår to the front, but you must remember the V2 rule, meaning the verb must stay in the second position. The sentence would become Igår såg vi ett nytt ljus på himlen, swapping the subject vi and the verb såg.