Learn Swedish: Är det en bevislig sanning?

Here, bevislig (provable) is an adjective describing the noun sanning (truth). You use the -ligen suffix to create a sentence adverb, which modifies the whole sentence rather than a specific noun. If you wanted to use the adverb, you would rephrase to Det är bevisligen en sanning (It is demonstrably a truth) or Det är bevisligen sant (It is provably true).

What is the root word of 'bevislig', and how does the suffix change its meaning?

The root is the verb att bevisa (to prove) or the noun ett bevis (a proof). Adding the suffix -lig turns it into an adjective meaning "capable of being proved" (provable). From there, adding -en creates the sentence adverb bevisligen (provably or demonstrably).

Is the 'g' in 'bevislig' pronounced the same way as it is in 'bevisligen'?

No. In the adjective bevislig, the g is usually silent in spoken Swedish, making it sound like "bevisli". However, when you add the -en to make the adverb bevisligen, the g is always clearly pronounced.

Could I use the adjective 'sann' (true) instead of the noun 'sanning' (truth) to express the same idea?