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"Against" is a preposition which is often translated as "contra", and "drop in" is an intransitive verb phrase which is often translated as "pasar". Learn more about the difference between "against" and "drop in" below.
against(
uh
-
gehnst
)A preposition is a word that indicates the relationship between a noun and another word (e.g. He ran through the door.).
1. (in opposition)
a. contra
Swimming against the current of the river takes much effort.Nadar contra corriente del río exige mucho esfuerzo.
b. en contra de
The people of the town were against the developer's plans.La gente del pueblo estaba en contra de los planes de la promotora inmobiliaria.
2. (in contact with)
3. (in comparison)
4. (in return for)
a. no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Delivery of the product is against presentation of the contract.La entrega del producto se hace al presentar el contrato.
We shall process the refund against your order once the paperwork reaches us.Procesaremos tu reembolso en cuanto nos llegue el papeleo de tu pedido.
5. (as protection from)
a. contra
Did you purchase insurance against fire for your new house?¿Contrataste un seguro contra incendios para tu casa nueva?
drop in(
drap
ihn
)An intransitive verb phrase is a phrase that combines a verb with a preposition or other particle and does not require a direct object (e.g. Everybody please stand up.).
1. (colloquial) (general)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
a. pasar
If we have time, we'll drop in after lunch.Si tenemos tiempo, pasaremos después del almuerzo.
b. dejarse caer (colloquial)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
Since they were in the neighborhood, they thought they would drop in on Marc and Guille.Como estaban en el barrio, se dejaron caer en casa de Marc y Guille.