"It is Monday today."
Translation:Det er mandag i dag.
If a "word" can be split into to meaningful components (without changing meaning), it should always be written as those components.
For instance you can write 'istedenfor'(instead of) because 'steden' isn't a word in Bokmål (any longer), but you'll have to write 'i stedet for', not 'istedetfor', because all of these are words.
Similarly you have to write 'i sted'(earlier) and 'i stad' not 'isted' nor 'istad', because while 'steden' isn't a word any longer, 'stedet' still is (the word has changed gender), and the indefinite form of these are the same, so 'sted' is a word.
If the word would change meaning when split into components, it should remain as one word. This is true for words like 'også'(too) because "og så"(and then) have another meaning, even if the word has been composed of those at one time. This is of course something you won't be able to tell without knowing.