Verbs ending in -ETER and -ELER such as geler, acheter, congeler ... are semi-regular -ER verbs in Présent indicatif, which means that, even though they take the regular -ER endings of Présent indicatif, their spelling and subsequently their pronunciation vary.
The majority of -ETER and -ELER verbs follow this rule: they double the T or L with je/tu/il/elle/on/ils/elles.
To practise these verbs, see the related lesson Conjugate -eter and -eler verbs in Le Présent - main rule (ll / tt)
HOWEVER with every rule come exceptions !
The following-ETER and -ELER verbs behave differently.
They always and only take the accent è on the first -e (-eter/-eler) with je/tu/il/elle/on/ils/elles (they can never double the T or L).
Here is the example of GELER (to freeze):
Of course, the pronunciation is affected: gèle/gèles/gèlent are pronounced [jell], whereas gelons/gelez are [juh-lon / juh-ley]
And here is the example of ACHETER (to buy):
Of course, the pronunciation is affected: achète/achètes/achètent are pronounced [achette], whereas achetons/achetez are [achuh-ton / achuh-tey]
Here is the complete list of exceptions, so you just need to know them (or at least the ones you might actually use (*), as most of the other ones are extremely rare!) :
agneler - to yean / give birth to a lamb
celer - to conceal
* receler - to conceal / to receive, hold stolen goods
ciseler - to chisel
* démanteler - to dismantle / take down
* écarteler - to quarter
s'encasteler - to be hoof bound
* geler - to freeze
* dégeler - to thaw (out)
* congeler - to freeze [something]
* surgeler - to quick-freeze [something]
* marteler - to hammer
* modeler - to model, to mould [US: to mold]
* peler - to peel (off)
* acheter - to buy
* racheter - to buy again, to acquire, to buy back
bégueter - to bleat (goat)
corseter - to corset
crocheter - to pick (a lock), to crochet
fileter - to thread
* fureter - to rummage, to nose about
* haleter - to pant
Look and listen at these other examples:
And to study other -E(-)ER and -É(-)ER verbs, see Conjugate -é(-)er, -e(-)er verbs in Le Présent (except -eter and -eler)