Gutturals

Gutturals (ח ה ע א) are consonants that due to being breathings have specific characteristics to them.
They cannot be doubled, therefore, they will not accept a daghesh forte. Due to this refusal, the preceding vowel typically heightens. This is referred to as a compensatory heightening. The vowel is changed to the long tone to compensate for the lack of the daghesh forte.

Resh ר

Resh ר is not a guttural and, therefore, cannot be doubled.

ה and ח

ה and ח are both doubled by implication. This means they will refuse a daghesh forte; however, since they are doubled by inference the following vowel is not heightened.

Compound Shewa

They prefer to use a vowel with a compound shewa. Typically, due to the peculiarities of the gutturals, the compound vowel used is אֲ Hateph Pathah. When the initial letter א it is found with a segol אֶ.

A Class Vowels

They prefer to use A class vowels.

Vowel Impact

They impact the vowels that precede them. When the vowel after the guttural is a shewa, the guttural impacts both the preceding and following vowels.
The preceding vowel becomes a hateph pathah ( ַ  ) unless the vowel must be maintained due to the characteristic identity of the form. The ר behaves in a similar manner to the guttural, preferring the pathah ( ַ  ) over the segol ( ֶַ  ) or kamets ( ַָ  ) .