These modes are called "tehtar" and represent different vowel sounds. They are used to indicate the vowel sound that follows the consonant.
Here are the seven tehtar:
1. anna (a)
2. anna-nuquerna (e)
3. anga (i)
4. anda (o)
5. an-nuquerna (u)
6. an-cume (au/aw)
7. an-nuquerna-cume (eu/ew)
It's important to note that the Tengwar is a consonantal alphabet, meaning it only represents consonant sounds. The vowels are indicated by the tehtar.