Questions & Answers about Silta on pitkä mutta turvallinen.
On is the 3rd person singular of olla (to be).
• silta on = the bridge is
You need it to link the subject to its description.
They’re used predicatively (after on), so they stay in the basic nominative singular form. Attributive adjectives (before a noun) must agree in case, but predicative ones do not.
• attributive: pitkä silta (long bridge)
• predicative: silta on pitkä (the bridge is long)