Learn Finnish: Tammikuussa jää on vahva. - In January the ice is strong.

The ending -ssa means "in" (the inessive case). Just like you use it for physical places (like talossa for "in the house"), Finnish uses it for months to say when something happens. So, tammikuu (January) becomes tammikuussa (in January).

What is the word tammikuu made of?

All Finnish months end in -kuu, which means "moon" or "month". The first part, tammi, means "oak tree" in modern Finnish, but in old Finnish it meant "core" or "center". So tammikuu is the "core month" of winter!

Is it normal to start the sentence with Tammikuussa instead of the subject jää?

Yes! While you can absolutely say Jää on vahva tammikuussa, Finnish often puts the time or place at the very beginning of the sentence to "set the scene" before introducing the main subject.

Why is vahva in its basic dictionary form here?

Because it describes jää (ice), which is the subject of the sentence and is also in its basic dictionary form (the nominative case). In Finnish, adjectives change to match the case of the noun they describe, so here they both stay in the basic form.