There were many types of grenades throughout modern warfare history such as stick grenades, impact, timed, stun etc. The first type grenade goes back over 1000 years in China, it was a simple container, with a fuse, which was filled with gun powder. This would be one of the first prototype to the modern day grenade.
Moderately effective due to trench warfare.
The Byzantine Empire used fire bombs vaguely similar to grenades as early as the late 700's AD. The first exploding grenades that were similar to modern ones were invented in China in the 900's.
The nuclear bomb was not used in trench warfare.
No and the Game's title of the November 2007 release is Call of Duty 4 Modern Warfare. If there is a Modern Warfare 4 which is doubtful since there are title confusing problems as a result of the number 4 being used in the name in the first game of the Modern Warfare series it will be released in November 2013
Grenades were used in warfare LONG before WW I. However, they were reserved for special uses. In WW I they were carried by the average combat soldier. Grenades were in use before the American Revolution- 141 years earlier than WW I.
The related link lists and described the weapons used in Call of Duty Modern Warfare 2
Buy Call of Duty: Modern Warfare first; you'll beome more familiar with the characcters and the weapons used before you get the second one. Besides, if you don't get the first one, the plot of the second one won't make much sense, will it? You'll have no idea what happened before. Also, when this answer was written, Call of Duty: Modern Warfare 2 wasn't released, so you can hardly get it this instant. Get Call of Duty: Modern Warfare first, and save up for Call of Duty: Modern Warfare 2.
yes
They were used in Vietnam by the Vietcong.
Surprisingly, grenades are older than guns- the first was probably from around the year 750 AD. Not 1750, but 750. See the link below.
Nothing happens other than getting the achievements and trophies in modern warfare 2. But in modern warfare 1, you get to unlock certain cheat codes that can be used during the game.
Yes, a keyboard is necessary to play Modern Warfare 2 on PC, however you cannot use a keyboard to play Modern Warfare 2 on PlayStation 3 nor Xbox 360.