Hebrew has no indefinite article. Indefiniteness is generally indicated by the absence of the article, קוֹל kôl, a voice. שֵׁם shēm, a name.

The definite article is always a prefix. It is joined to the word so closely as to make one word in writing and pronunciation. It suffers no change for gender or number.

The usual way of writing the article is to prefix הַ to the noun and put a daghesh forte in the first consonant.

Gutturals א ,ע, and ר refuse the daghesh forte, and hence the vowel must be heightened because the otherwise short vowel of the article would be left in an open syllable unaccented.
The normal thing is to heighten the ַ  to ָ  .
Example: הָאָב הָרֹאשׁ.

They are as follows:
אֶ֣רֶץ, érĕts, earth with the article becomes הָאָ֣רֶץ hä’ärĕts.
עַם ăm, people, becomes הָעָם hä’äm, the people.
הַר hăr, mountain, becomes הָהָר hähär, the mountain.
These syllables are accented, and the vowel is at liberty to change its length for the sake of harmoniousness.