It can be both as in English both "kanaal" and "gracht" are translated to canal.
There is a difference between the both in Dutch though. "Kanalen" and "grachten" are different in the way that you'll only find a "gracht" in or around a city or castle and in that sense are only suitable for local transportation and mainly served for defending the city/castle. A canal generally connects areas over a long distance and were build to transport goods over a large distance.
That doesn't mean though that a city cannot have multiple "kanalen". To take Amsterdam as an example. Amsterdam obviously has loads of "grachten" which create circles around the city centre, so local transport and defence. But Amsterdam also has multiple "kanalen", e.g. the Amsterdam-Rijn Kanaal and the Noordzee Kanaal.
As I remember it, there was a very useful set of hints and tips in the plurals module and I've picked up more from discussion threads - as this one:
There are also a number of nouns that end with -s in the plural, because they either end a certain way or are borrowed words. The following nouns get the ending -s in the plural: ¥ Nouns with two or more syllables ending on an unstressed -el, -em, -en, -er, -erd, -aar, -aard, and all diminutives: ¥ de tafel (table) -> tafels ¥ het modem (modem) -> modems ¥ de jongen (boy) -> jongens ¥ de vader (father) -> vaders ¥ de stommerd (dummy) -> stommerds ¥ de adelaar (eagle) -> adelaars ¥ de luiaard (sloth) -> luiaards ¥ Most loanwords that end in a consonant, which also end in s in the language of origin: ¥ de film (film/movie) -> films ¥ de computer (computer) -> computers ¥ de roman (novel) -> romans ¥ Nouns ending in an unstressed -ie: ¥ de familie (family) -> families ¥ de organisatie (organization) -> organisaties ¥ Exceptions to this are “de bacterie” (bacteria) -> “bacteriën” and “de porie” (pore) -> “poriën” The ending ’s (apostrophe s) is added to words ending in -a, -i, -o, -u or -y. This is necessary because otherwise the vowel length would change (think of the spelling rules): ¥ de firma (firm) -> firma’s ¥ de taxi (taxi) -> taxi’s ¥ de auto (car) -> auto’s ¥ de paraplu (umbrella) -> paraplu’s ¥ de baby (baby) -> baby’s