Questions & Answers about Poolen är femtio meter lång.
Why is "meter" not in a plural form like "metrar" after "femtio"?
In Swedish, most units of measurement that are en-words, like meter, mil, or liter, have the exact same form in the plural. So one meter is en meter, and fifty meters is femtio meter.
Since "femtio meter" is a plural amount, why is the adjective "lång" in the singular form instead of "långa"?
The adjective lång describes the subject of the sentence, which is poolen (the pool). Because poolen is a singular en-word, the adjective must also be in its basic singular form. The measurement femtio meter just acts as an adverbial phrase telling us the extent of that length, so it doesn't make the adjective plural.
How do we know whether an English loanword like "pool" is an en-word or an ett-word?
Most new loanwords that come into Swedish become en-words by default. Since pool is an en-word, it takes the standard -en ending to become the definite singular poolen (the pool).