I'm new in forex trading. I use MetaTrader 4, but I don't understand something:
Ex: I set Buy at 0.7780, Target: 0.7900, Stop: 0.7720(AUD/USD), so I set take profit at 0.7900, but when it reach 0.7900 the order hasn't stopped. Why?

Thanks in advance!

Where does the investor place his stop and limit orders respectively, determines the amount of risk he is taking up. It is advisable not to place your stop/loss orders too close to the normal market price, as a little fluctuation in the market, can then trigger the order. Likewise, limit orders should also reflect a rational hope of profits you are expecting, based on the market's trading activity. They should be set at the rate which is not overexposed to the trade, and also not too close to the market.
'Stop-loss' and 'limit' orders can lower an investor's exposure to risk by a large proportion.

Double check your take profit rate to make sure it was set at that point, if so I recommend contacting the broker to ensure that future take profit & stop loss rates function correctly as these rates are a key part in managing risk.

ANYWAYS, to answer your question instead of jibber jabber..

right click on screen and click properties then goto common and check box SHOW ASK LINE.
Then you can see the spread.
The spread is why your trade didn't get closed when you thought most likely, unless you had TP set incorrectly or something

You need to take the spread into account. When you open a buy order, the BID price is the one applicable.
If the price went past 0.7900 and the order still didn't close on target, you should find a new broker.

thats a good piece of advice. Proper money management will indicate your stamina in the game. If you follow a breakout method, a trade can go well and fast in your favor or snap back and go back on you.

You are right! money management plays an important role no doubt.

there are three price there
the price bank sell to u
market
the price bank buy from you

you want to close your order means that you need to sell them to bank
so it is not the price of market but the price bank buy from you

so the price is low than 0.7900

so if the market price up to 0.7910
then your order close