Genesis 2:16-17 (NASB)
The LORD God commanded the man, saying, "From any tree of the garden you may eat freely; but from the tree of the knowledge of good and evil you shall not eat, for in the day that you eat from it you will surely die."
At first glance, it is difficult to see how this was true, according to the Genesis narrative. After all, the day that Adam and Eve ate from the forbidden tree, they didn't fall down dead. Instead, Genesis tells us that they continued to live and had many children afterwards.
However, with any ancient, translated text it is important to remember that the modern English translation never does perfect justice to the original. In this case, the original was written in ancient Hebrew, not modern English.
Before we jump to conclusions, surely it would seem odd that the individual who put Genesis together would portray his God in such a way as to be completely wrong about something such as this, and provide no commentary whatsoever on the situation... no justification or explanation as to why His God made such an error. It seems far more probable that what was meant by the author is for some reason lost in the translation.
The New American Standard Bible (NASB), the version quoted above, is about as close to a literal word-for-word translation of the Bible to modern American English as we can currently find. Most of the time, this is a good method of translating for study purposes, but many people find the NASB a bit less flowing and more difficult to read than a thought-for-thought version like the New Living Translation (NLT).
Imagine trying to translate the following statement into a foreign language:
"That was a piece of cake -- I could do it in my sleep."
A word-for-word translation would tend to render each word literally, telling readers that the speaker's piece of cake could be done in his sleep, a statement that would have absolutely no coherent meaning in any other culture or language.
A thought-for-thought translation would be more likely to take the meaning of the statement, rather than the exact words, and render that meaning in the words and context of the receptor language. It would probably come out kind of like the following:
"That was so easy -- I could do it in my sleep."
Indeed, when we read the New Living Translation (NLT) version of the same scriptures:
But the LORD God warned him, "You may freely eat the fruit of every tree in the garden -- except the tree of the knowledge of good and evil. If you eat its fruit, you are sure to die."
This version certainly makes a lot more sense given the rest of the Genesis narrative, but is it really a legitimate translation of the text? Based on my research, it appears as though the Hebrew can carry the meaning:
"...in that day, your death will be sure."
In other words, it seems to me that the text has God saying that Adam and Eve guaranteed their own deaths on the day that they ate from the tree, and not that God is saying that they would actually fall down dead on that very day.
More on Genesis 2:17
I can only assume you are speaking about the Biblical account of the creation story, because in the Qur'an He told them that if they ate from the tree they would be wrongdoers (2:35). After eating from the tree, they were expelled from Paradise and told that Earth would be their dwelling place for a time (2:36). Thereafter, God forgave them of their sin (2:37).
Perhaps what is intended from the passage you read was that they would be mortal, instead of living in Paradise forever.