El and un are always used immediately before singular feminine nouns beginning with stressed a- or ha.

Here are some examples:

el agua – water
el/un águila – eagle
el/un alba – dawn
el/un alma – soul
el/un alza – an increase
el/un ancla – anchor
el/un área – area
el/un arpa – harp
el asma – asthma
el/un aula – classroom
el/un hacha – axe
el/un hada – fairy
el/un hambre – hunger
el hampa – underworld (crime)
el haya – beech (tree)

Tiquismo of the week:

Jumas or jumo means drunk in Costa Rica. Jumarse means to get drunk