Learn Swedish: Guiden visade all konst. - The guide showed all the art.

The word konst (art) is an en-word. When we want to say "all" or "all the" with an uncountable en-word, we use all. If it were an ett-word, we would use allt, and for plural nouns we would use alla.

The English translation says "all the art". Why doesn't the Swedish sentence use the definite form konsten?

This is a major difference between Swedish and English! After the words all, allt, and alla, Swedish uses the indefinite (base) form of the noun. So "all the art" translates to just all konst, never all konsten.

How is the word guiden formed?

It is a loanword that acts like a regular Swedish en-word. The base word for "guide" is en guide. To make it "the guide", you just add the definite ending -n to get guiden.

What tense is the verb visade?

It is in the past tense, meaning "showed". The infinitive is att visa (to show). Because it is a regular Group 1 verb, it forms its past tense by simply adding -de to the stem.