Difference between «ЗА» and «ДЛЯ»

The difficulty can appear for Russian learners when deciding what preposition to use - «за» or «для», in cases when preposition «за» is translated in English as «for». «Для» can always be translated as «for».

Let’s take a look at three cases when Russian expressions with preposition «за» are translated in English with preposition «for»:
идти за молоком - to go for milk
посылать за доктором - to send for a doctor
платить за билет - to pay for a ticket

Tips for similar cases:
Preposition «за» is used together with means by what you want to reach what you wish.

Preposition «для» is used to indicate a subject for whose benefit, for whom or in whose interest this action is done.

Therefore, we can say:
идти за молоком для мамы - to go for milk for my mom
milk is means to make mom happy
посылать за доктором для сына - to send for a doctor for my son
doctor is means to make son happy
платить за билет для проезда - to pay for a ticket for a ride
ticket is means to make a ride

Let’s consider one more case.
Покупать за деньги. - To buy for money.
In Russian variant preposition «за» is used because you are buying not for benefits of your money, and not for interests of your money, you are buying for your husband (для мужа) or your son (для сына) in exchange for money («за деньги»), you are using money as a means to purchase something for the benefit of someone.