"Mager" has three common translations related to body volume, quality or food.

1. slim, thin, skinny [adjective/adverb]

This is the most common translation of "mager": the opposite of "fat". We also say "dun" ("thin"), but "mager" is ‘thinner than thin’ and is more associated with not eating enough.

Examples:
– "Een collegaatje van mij is zo mager, we vragen ons af of ze wel goed eet!"
("A (female) colleague of mine is so thin, we wonder whether or not she eats properly!")

– "Veel sporten is goed voor de lijn, maar je moet niet te mager worden."
("Exercising often is good for the figure, but you shouln’t become too skinny.")

– "Kate Moss dun? Mager zal je bedoelen!"
("Kate Moss thin? You must mean skinny!")

Related words:
– Broodmager: scrawny [adjective].
– Dun: thin [adjective/adverb].
– Dik: fat [adjective/adverb].
– Afvallen: to lose weight [verb] [viel af, afgevallen].
– Aankomen: to gain weight [verb] [kwam aan, aangekomen].
– Dieet: diet [noun] [het dieet, de diëten].

2. feeble, weak, poor [adjective/adverb]

You can also use "mager" to indicate low quality or poor performance.

An interesting development in the Dutch language is that we sometimes create an informal version of an adjective by turning it into what I can best describe as a "plural diminutive". Which is weird for an adjective! 🙂 So in this case you could say that something is "magertjes". See the examples. You might also hear it applied in the first translation above. Other examples of this phenomenon are "zachtjes" (quietly) or "eventjes" (for a short while).

Examples:
– "Ik vond de bijdrage van Frank wat magertjes. Normaal gesproken levert hij betere kwaliteit!"
("I found Frank’s contribution quite poor. Usually he provides better quality!")

– "Wat een mager excuus, hij wist toch al lang dat hij dat weekend vrij moest houden in zijn agenda!"
("What a feeble excuse, surely he has known for a long time that he was supposed to block that weekend off in his calendar!" Note that Dutch people use the word "agenda" for a diary, organizer or (pocket) calendar. So don’t be sruprised when they go and "check their agenda". The Dutch like to use the English word "weekend" but we have a perfectly acceptable Dutch word for that which is "weekeinde".)

Related words:
– Matig: moderate, mediocre [adjective/adverb].
– Kwaliteit: quality [noun] [de kwaliteit, de kwaliteiten].

3. lean, meagre [adjective/adverb]

You might prefer "skimmed" milk over "semi-skimmed" milk. In the Netherlands you would then buy "magere melk" instead of "halfvolle melk". Note that "halfvol" literally means "half full". If a Dutch person ever asks you about "full milk", then probably he or she means "whole milk".

Regarding food you might hear "magere kost" ("lean fare") or "mager vlees" ("lean meat").

wine [noun] [de wijn, de wijnen]

The Dutch word for wine is similar to the English: "wijn". The dimunitive of "wijn" is commonly used to refer to a glass of wine: "een wijntje".

Examples:
– "Ik ga vanavond lekker in de zon zitten met een glas wijn."
("I’m going to sit in the sun tonight and enjoy a glass of wine".)

– "Ter ere van zijn promotie hebben we een wijnproeverij georganiseerd."
("In honour of his job promotion we organised a wine tasting.")

– "<Ober:> De huiswijn serveren we per glas of karaf; de andere wijnen worden per fles geserveerd."
("<Waiter:> We serve the house wine per glass of carafe; the other wines are served per bottle.")

– "Prosecco is een mousserende witte wijn."
("Prosecco is a sparkling white wine.")

– "Ik heb een kater van die wijn van gister: wat een Chateau Migraine!"
("I have a hangover due to that wine yesterday: wat a god-awful wine!" Please note that chateau migraine is a joke referring to a fictive French vineyard literally translated as "Castle Migrain".)

– "De serveerster struikelde en liet het dienblad met wijnglazen in de gracht vallen."
("The waitress tripped and dropped the tray with wine glasses in the canal.")

– "Wat wil je drinken? Een wit wijntje, een rood wijntje of een rosé’tje."
("What would you like to drink? A glass of white wine, a glass of red wine or a glass of rosé wine?")

Expressions:
– "Bier na wijn geeft venijn; wijn na bier geeft plezier."
("Drinking beer after wine will make you feel ill; wine after beer will give pleasure".)

Oddly, there is no agreement in Europe about the truth of this saying. The Germans have a saying that agrees with the Dutch saying, but the British tend to reverse it:
"Beer after wine, and you’ll feel fine, wine after beer and you’ll feel queer".

Related words:
– Wijngaard: vineyard [noun] [de wijngaard, de wijngaarden].
– Wijnproeverij: wine tasting [noun] [de wijnproeverij, de wijnproeverijen].
– Wijnglas: wine glass [noun] [het wijnglas, de wijnglazen].
– Wijnazijn: wine vinegar [noun] [de wijnazijn, no plural].

friend [noun] [de vriend, de vrienden]

The translation is obvious and the usage is obvious, however… This noun is a typical example of a noun of which the diminutive has a specific meaning. "Vriendje" is used for a friend of a small boy (if also small of course), but also for a "boyfriend"… The same is true when "vriend" is used with a possessive pronoun, e.g. "mijn".
If you want to avoid strange looks don’t say "dit is mijn vriend",  when you mean to say "this is a friend of mine", but say "dit is een vriend van mij" instead. Note that in fact "dit is mijn vriend" translates to "this is my friend" so grammatically there is no issue 🙂

This subtlety also exists with the female version: "vriendin".

"Vriend" can also be used in the meaning of "mate" or "dude". See also DWOTD Makker.

Examples:
– "Mag ik je aan mijn vriend voorstellen?"
("May I introduce you to my boyfriend?")

– "Ik durf het bijna niet te vragen, maar ben jij nu mijn vriendje?"
("I’m a bit shy to ask, but are you now my boyfriend?")

– "Dit is een vriend van mij. Ik ken hem nog van vroeger."
("This is a friend of mine. We’ve been friends since we were young / We’ve been friends for a long time." Literally: I know him from the old days.)

– "Dit is mijn vriend. Eh, ik bedoel, dit is een vriend van mij."
("This is my (boy)friend. Err, I mean, this is a friend of mine.")

– "Als je wint, heb je vrienden."
("When you win, you have friends." Line from the song "Als je wint" by "Doe Maar", a famous Dutch eighties ska/reggae/pop band.)

– "Op deze manier maak je geen vrienden gast, ik zou maar even dimmen als ik jou was!"
("You don’t make yourself popular this way mate, I would take it a little easier if I were you!" Literally: "This way you don’t make friends…")

Related words:
– Vriendelijk: friendly [adverb/adjective].
– Vriendschappelijk: friendly, amicable [adverb/adjective] (as in "a friendly match").
– Maat/maatje/makker: popular slang synonyms for "vriend" [noun] [de maat/het maatje/de makker, de maatjes/de makkers].
– Vriendschap: friendship [noun] [de vriendschap, no plural].

Example:
– "Een keer trek je de conclusie, vriendschap is een illusie."
("One time you will draw the conclusion, friendschip is an illusion." Famous line from the song "Vriendschap" by the Dutch eighties band "Het goede doel".)

Extra:
Toon Hermans was a Dutch comedian who was very popular in the fifties and sixties. He wrote the well-known poem "Vriend". You might see it on a "wc-tegeltje" if you visit elderly Dutch people. It goes:

"Je hebt iemand nodig, stil en oprecht,
Die als het erop aankomt, voor je bidt of voor je vecht.
Pas als je iemand hebt, die met je lacht en met je grient,
Dan pas kun je zeggen, ik heb een vriend."

Some help on the translation: "oprecht" translates to "sincere". The verb "grienen" is a synonym for "huilen", but is not used much anymore.