Linking German articles to a specific gender or concept can be quiet tricky. Although there are a few rules or better said guidelines, there are many exceptions that make it more confusing.

Your best bet is to learn the gender of an article as part of the noun itself.

Regardless, here are some guidelines you can follow:

DER

- weather phenomena: der Regen, der Nebel, der Sonnenschein, der Schnee, der Hagel

- calendar words and seasons: der Monat, der August, der Tag, der Montag, der Sommer. Exception is here: die Woche (ending in -e), das Jahr and das Frühjahr (because of das Jahr)

- all words ending in -ismus: der Journalismus, der Kapitalismus

- all words ending in -(n)er: der Lehrer, der Schüler, der Kellner

- Agents (the ones who do the action) or professions: der Arzt, der Postbote, der Architekt, etc. (there is the feminine form of this almost always ending in -in but with variations in the vowels: die Ärztin, die Postbotin

- mountains (der Mount Everest), most rivers outside Europe (der Tiber, der Mississippi)

- most nouns ending in -ich, -ling, -ist, -isch: der Strich, der Frühling, der Alchimist, der Hirsch

DIE

- Nouns ending in: -heit, -keit, -tät, -ung, -schaft - 

die Freiheit, die Ewigkeit, die Universität, die Zeitung, die Freundschaft 

These suffixes usually have a corresponding English suffix, such as -ness (-heit, -keit), -ty (-tät), -ship (-schaft).

- Nouns ending in -ie: Drogerie, Geographie, Komödie, Industrie, Ironie (often equal to words ending in -y in English)

- Names of aircraft, ships and motorbikes: die Boeing, die Titanic. The die comes from die Maschine, which can mean plane, motorbike and engine. - Helpful reminder: Ships are often referred to as "she" in English.

- Nouns ending in -ik: die Grammatik, Grafik, Klinik, Musik, Panik, Physik

- Borrowed (foreign) nouns ending in: -ade, -age, -anz, -enz, -ette, -ine, -ion, -tur: Parade, Blamage, Bilanz, Distanz, Frequenz, Serviette (napkin), Limonade, Nation, Konjunktur.

Such words often resemble their English equivalent. A rare -ade exception: der Nomade.

- Nouns ending in -in that pertain to female people, professions, nationalities: Amerikanerin, Studentin, but der Harlekin and also many non-people words: das Benzin, der Urin.

- Most nouns ending in -e: Ecke, Ente, Grenze, Pistole, Seuche, but der Deutsche, das Ensemble, der Friede, der Junge.

- Nouns ending in -ei: Partei, Schweinerei, but das Ei, der Papagei.

- Most types of flowers and trees, except der Baum

- Borrowed (foreign) nouns ending in -isse, -itis, -ive: die Kulisse, Dermatitis, Initiative.

DAS

- Nouns ending in -chen or -lein: Fräulein, Häuschen, Kaninchen, Mädchen 

These suffixes are also to express something like "little" in a sense of cute, like "little, cute dog" would be das Hündlein or Hündchen. Note the change of the vowels in many words.

- Infinities used as nouns (gerunds): das Essen, das Schreiben

- Metals: das Silber, das Gold, exception: der Stahl

- Colors used as nouns: das Gelb, das Grün

- places with adjectives: das moderne Deutschland, das koloniale Amerika, das frühe Mittelalter

- Nouns ending in -o: das Auto, das Büro, das Kino, exception: der Euro

- Most nouns starting with Ge-: Geschirr, Geschlecht, Gesetz, Gespräch, but there are many exceptions, such as der Gebrauch, der Gedanke, die Gefahr, der Gefallen, der Genuss, der Geschmack, der Gewinn, die Gebühr, die Geburt, die Geduld, die Gemeinde, die Geschichte, and others.

- Most borrowed (foreign) nouns ending in -ment: Ressentiment, Supplement (but der Zement, der/das Moment [2 diff. meanings])

- Most nouns ending in -nis: das Versäumnis, but die Erlaubnis, die Erkenntnis, die Finsternis

- Most nouns ending in -tum or -um: Chrsitentum, Königtum, but der Irrtum, der Reichtum 

As you see, there are a lot of exceptions to the rules or guide lines to this and therefore my suggestion to memorize the article together with the noun.

Let me know if you have another question!

I hope you had time to "digest" this information?

Based on the rules above you can try to find the article for those words:

- Einigkeit

- Institutionalismus

- Demokratie

- Klempner

- See

- Anarchist

- Emotion

- Ozean

- Idee

Also, very important to keep in mind: German language loves to have combined words, such as "das Einbürgerungsamt": die Einbürgerung (naturalization), das Amt (department, agency) = department of naturalization. Note that the article of the combined word is the one of the last word (das). The rule always is here that the last word in the combined word determines the article of the whole structure.

Find the article for those combined words:

- Hundefutter (Hund, Futter)

- Lebensmittelladen (Lebensmittel, Laden)

- Winterkleidung (Winter, Kleidung)

Have fun! :)