Michael Swan-What Grammar Does that Lexis Alone Can't

In short, from the linguist's point of view

• expressing relationships between things-spatial, causal, temporal
• distinguishing between stating and asking 
• modality
• relating concepts

From a teaching point of view

• Prioritise-don't go beyond what is needed
• Useful, essential, unimportant
• easy, difficult, impossible

Check out the first part of this link.

http://www.livesofteachers.com/2010/12/02/an-interview-with-michael-swan/