I get frustrated when i make these stupid mistakes in my English classes, though i know grammatical rules well, for example when i want to talk i talk like this "yesterday i ran into my sister i told HIM i WERE going to a movie ..." I've been studying English for more than four years and i have my sights set on learning English but i've failed to come up with a solution to this problem. My teachers say it's because first i think in my native language ( Farsi ) then i translate what i want to say into English and then i say it. If they're right, How can i stop it??

if u want use corect verb possesive adverb or some features yo have to to stop speaking pearsion and start to find some forign friends but if u live in iran take it easy man you can repeat your sentences every night befor sleep and find corect form of use i think your practice isnt enough
i wand to tell you some thing when i translate eng to persian i can speak beter so go and fu.. your teacher
that manner of learning english is for children becuase they have not lots of experience in persian and so they can imagine a shape of apple when they think about that but im sure u cant do that so just speak alot and find your problems and mistakes and say correct sentence again.

it only means that your foreign language level is not high yet... when you use it often enough there is no "translation" from native to foreign,... it's just foreign words appear in your head and make fluent sentences... it can be stopped by regular and persistent practice

I am welsh and i do the same infact im doing it now,

I live in a country where English is also spoken and i still only think in Welsh , but the weird think is when i go to south africa i can fink in afrikaans without having to translate from Welsh but i cant do it in English?

Everyone learning a foreign language is experiencing the exact same thing as you! It is normal for you to think and then speak at the beginning process of language learning. The only way you are going to be able to "not think before you speak" is if you are around the language continuously in a place where English is spoken. The more you speak and hear it, the more you will become comfortable with the language and use it subliminally. You'll be fine...don't worryjust find a way to be around the language as often as possible, maybe watch more English TV, movies, etc. or the best thing to do is get an English penpal or visit an English speaking country for a while. Submerse yourself around the language. This will help!