The realization of grammar shows on making higher level of language element called clause. Most students are  familiar with sentence. Actually, sentence is formed by clause. One clause reflects simple sentence. This fact should be internalized tightly in mind. How do we encounter when a sentence shows multiple processes? That is the reason of making clause as the base of language realization.
A clause may express whether presenting action, existence, or relation. It can be act of saying and perception (e.g she said (that) the man didn't do anything, the investigator thought the suspect had fled from the building ). One sentence may refer two different events which can be projected.
The multiple events happening on single sentence make the concept of clause as process like Halliday (1990) followed by Gerot and Wignell (1996), Thompson (2006), Martin (1992), and other functional grammarians concern. Learning English Grammar on Intermediate Level is required to master this concept. After learning basic part of speech, this is the time for expressing the sequence of constituent (the elements) all together into logical relationship.
It starts on complex sentence. This lesson leads students that an idea can be expanded. Of course, tense aspects must be strengthened firstly. Tense aspects are the bases of English expression on timing and context.
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Markers
Expanding ideas require markers. Complex sentence is marked so that users know the process is projected, added, and linked by another process outside from the main clause (the core idea before it's expanded). Although the event is outside from the main, there is circumstance surrounding the core.
"Take the umbrella in case it should be rain"
"He wrote the letter in the way that I asked "
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