In English, all that separates Dan from a Dane and Jan from Jane is a shorter vowel sound. Similarly, in German, a Beet (flower bed) becomes a Bett (bed) as the vowel shifts from long to short. Mastering this distinction starts with recognizing that both long and short vowels exist and learning to tell them apart.
German vowels fall into four distinct categories: standard, modified, combined, and reduced vowels. The five standard vowels in German—a, e, i, o, and u—are written the same as in English. Each has both a long and a short version. Modified vowels are marked by two small (but impactful) dots above them: ä, ö, and ü. As you can tell, they're variations of the standard vowels also come in long and short versions. Combined vowels are a different animal: they are blends of two vowel sounds. These are always pronounced long, as two vowels naturally take more time to speak. Reduced vowels, on the other hand, occur only at the end of a word and are usually pronounced short.
Double vowels, referring to two consecutive identical vowels, always produce a long vowel sound. This spelling is an intuitive way to indicate a long vowel sound. Examples include: Aal (eel), Boot (boat), See (lake), and Tee (tea). Notably, this concept also exists in English, as illustrated by the word pairs bleed vs. bled, feed vs. fed, and root vs. rot.
Another common marker of a long vowel is the letter h. This spelling dates back to earlier forms of German, where many words featured an audible h sound after a vowel. Over time, the h sound was lost, but the spelling remained to indicate the long vowel. Examples include the words Bahn (train), Huhn (chicken), Lohn (wage), and Zahn (tooth).