Inferno can be a tricky map to defend unless you understand it. To that end, start by learning the ‘call-outs’—the names used to describe each part of the map. This guide by Froosh and collaborators is a very useful resource to bookmark. Keep the Inferno image open in a tab as you work through the rest of this article.
There are two major chokepoints that you want to control: middle and top of banana. If you manage to keep the terrorists from pushing past these points until the end of the round you are likely to have gathered enough information to be ready for the final aggression.
At first glance this may seem simple, but if you have played a couple of games on Inferno you know that this isn't the case. One reason for this is the long rotations between the two bomb sites. As a consequence, you will have to slow down the terrorists.
Always prioritize smoke grenade, then two flashbangs. If you have money left you should go for an incendiary unless you plan to go for double grenades in either banana or middle to do some initial damage.
Having identified the problem, it's time to defuse it. In order to have a successful CT half you have to make the most of your utility grenades: don't throw away a flashbang for no reason.
You want two guys to cover mid. Preferably one on arch side and one truck side. These guys will be your impenetrable wall. They will use their smoke grenades to ensure the terrorists won't be able to take over mid (at least not without begging for mercy when the rotation from B joins the party.)
You can use a flashbang to let your teammate peek and maybe even get a kill. If that player finds terrorists about to push mid he mustn't panic. Instead, you should just put down a smoke screen. Generally the player on the arch side should use his smoke first. That way you will still have smokes left on A if the arch player has to rotate to B.
The third player on A will make sure your opponents don't take control over apartments. This guy will be the James Bond of your team: the secret agent whose aim is to go for sneaky plays and catch the terrorists off-guard. If you have a smoke up in mid, the truck side player can support him from boiler stairs and let him hide in bedroom.
On the next page: control bombsite B, plus when to buy an AWP.