"Haar" is a common or a neuter noun, depending on the meaning. Note that "haar" can also be a possessive or personal pronoun, see 3. and 4. below.

1. hair
[noun]
[het haar, <no plural>]

You say "het haar" when talking about hair in general (such as body hair) or your haircut/style/do.

Examples:
– "Mijn haar ziet er niet uit, sorry, maar zo kan ik echt niet uit." 
("My hair looks really bad, I’m sorry, but I can’t possibly go out like this.")

– "Wat zit je haar leuk!" – "Nou, dankjewel!" 
("Your hair looks really nice!" – "Well, thank you!")

– "Sander heeft gisteravond zijn haar gemillimeterd." 
("Sander trimmed/cropped his hair last night." Here the verb "millimeteren" is obviously derived from the length of the hairs left after trimming.)

– "Haar haar zit leuk, vind je ook niet?" 
("Her hairstyle is really nice, don’t you think?")

– "Is jouw haar geverfd, of is dat je echte haarkleur?" 
("Is your hair dyed, or is that your real hair colour?")

Expressions:
– "Als je haar maar goed zit…": literally "as long as your hair looks fine…" this says something like "why worry over something…"

Related words:
– Kapsel: hairstyle/do [noun] [het kapsel, de kapsels].
– Haarstijl: hairstyle [noun] [de stijl, de stijlen].
– Borst/been/schaam/okselhaar: chest/leg/pubic/armpit hair [noun] [het haar, <no plural>].
– Kaal: bald [adjective].

2. hair
[noun]
[de haar, de ha-ren]

You say "de haar" when talking about a single hair, or "de haren" in case you mean a collection of single hairs as opposed to a hairdo.

Examples:
– "Van al dat harde werken krijg je grijze haren als je niet oppast." 
("All that hard work may cause grey hairs (to appear) if you don’t watch out.")

– "Van wie zijn die blonde haren?" – "Ik heb geen idee, echt waar schat…" 
("Whose blonde hairs are those?" – "I haven’t got a clue, honestly darling.")

– "Er zit een haar in mijn salade." – "Bah, wat vies!"
("There is a hair in my salad." – "Yuck, how disgusting!")

Expressions:
– "Daar krijg je grijze haren van": that will turn your hair grey.
– "Op een haar na": very nearly.

Example:
– "De auto raakte de voetganger op een haar na."
("The car very nearly hit the pedestrian.")

– "Geen haar op mijn hoofd die daar aan denkt": I would not dream of it. Lit.: "Not a single hair on my head that will think of that".

Related words:
– Behaard: hairy [adjective].

3. her
[possessive pronoun]

Example:
– "Ik zweer het, dat is mijn haar, niet haar haar!" 
("I swear, that is my hair, not her hair!")

4. her
[personal pronoun]

Example:
– "Ok, ik zal het haar zeggen." 
("OK, I will tell her.")

toasted ham and cheese sandwich
[noun]
[de tos-ti, de tos-ti’s]

There’s not really an equivalent in English for “tosti”. In French, there is, though: “croque-monsieur”. “Tosti” apparently originates from the Italian “tosto” (from which “tosti” is the plural form), which is short for “tostato”, which is the past particple of “tostare”: to toast.
(Photo: Heather-D (flickr.com) – some rights reserved.)

A “tosti” is two slices of bread, by default with cheese and ham in between,
toasted all together. The ingredients may vary, but cheese (of any
kind) seems to be mandatory. If you order a tosti without further
specification, you will receive a ham and cheese tosti, mostly served with a small bowl of ketchup.

Examples:
– “Ik wil graag een tosti.”  – “Ham en kaas?” – “Ja, graag.”
(“I would like to have a tosti.” – “Ham and cheese?” – “Yes, please.”)

– “Lara maakt de allerbeste tosti’s!! Met Franse salami, tomaatjes en oude kaas…”
(“Lara makes the absolute best tostis!! With French salami, little tomatoes and old cheese…”)

– “Ober, mag ik mayonaise bij mijn tosti in plaats van ketchup?” – “Vanzelfsprekend, meneer.”
(“Waiter, can I have mayonaise with my tosti instead of ketchup?” – “Naturally, sir.”)

Related words:
– Tosti-apparaat: toaster device (for making tostis) [noun] [het tosti-apparaat, de tosti-apparaten].
– Tosti-ijzer: toaster device (for making tostis, lit.: tosti iron) [noun] [het tosti-ijzer, de tosti-ijzers].
– Broodrooster: toaster (for toasting slices of bread) [noun] [het broodrooster, de broodroosters].
– Roosteren: to toast [verb] [roosterde, geroosterd].

Extra:
You have a “broodrooster” (toaster) and a “tosti-apparaat/ijzer”. They are different devices. The former is a device used to toast bread, especially by exposure to electrically heated wire coils. The latter is a device for making tostis by heating/toasting a ham/cheese sandwich between two metal plates. These plates are usually heated electrically.

to practise sorcery / witchcraft, to conjure (up)
[verb]
[to-ver-de, ge-to-verd]

“Toveren” is often used in its literal sense: witches and sorcerers do it all the time 🙂 It can also be used when someone does something which seems impossible.

Examples:
– “De tovenaar leerde de toverspreuken uit zijn hoofd.” 
(“The sorcerer learned the magic charms by heart.”)

– “Harry Potter gaat naar de school van tovenarij Zweinstein.” 
(“Harry Potter attends the school for witchcraft Hogwarts.”)

– “Toen ik vroeg of zij een tissue had toverde zij een hele rol keukenpapier te voorschijn uit haar tas.” 
(“When I asked her for a tissue, she conjured up an entire kitchen roll out of her purse.”)

– “Wil je binnen 10 minuten een artikel van twee pagina’s? Onmogelijk! Ik kan niet toveren.” 
(“Do you want a two-page article within 10 minutes? Impossible! I’m no wizard.”)

– “Wie heeft van de toverdrank gedronken? Asterix of Obelix?” 
(“Who drank (of) the magic potion? Asterix or Obelix?”)

– “Hans Klok is een van de bekendste Nederlandse illusionisten/goochelaars.”
(“Hans Klok is one Holland’s most famous illusionists.”)

Related words:
– Goochelen: to conjure, to perform conjuring tricks [verb] [goochelde, gegoocheld].
– Goochelkunst: conjuring trick [noun] [de goochelkunst, de goochelkunsten].
– Goochelaar: illusionist [noun] [de goochelaar, de goochelaars].
– Tovenaar: sorcerer [noun] [de tovernaar, de tovenaars].
– (Tover)heks: witch, sorceress [noun] [de toverheks, de toverheksen].
– Toverspreuk: magic spell, charm [noun] [de toverspreuk, de toverspreuken].
– Toverkol: witch, hag [noun] [de toverkol, de toverkollen].
– Toverdrank: magic potion [noun] [de toverdrank, de toverdranken].

Extra:
Dutch singer Herman van Veen – who slightly resembles a sorcerer himself – had a hit in the eighties with his song “Toveren“.