There are four basic types of construction in a complex verb phrase:
Type A (MODAL) consists of a modal auxiliary + the base of a verb: eg: must
examine.
Type B (PERFECTIVE) consists of the auxiliary HAVE + the -ed participle of a
verb : eg: has examined. (Traditionally the term PERFECT has been frequently
used instead of PERFECTIVE.)
Type C (PROGRESSIVE) consists of the auxiliary BE + the -ing participle of a
verb : eg : is examining.
Type D (PASSIVE) consists of the auxiliary BE + the -ed participle of a verb:
eg : is examined.
These four basic constructions also enter into combination with each other:
AB : may have examined
AC : may be examining
AD : may be examined
BC : has been examining
B D : has been examined
CD : is being examined
ABC : may have been examining
AB D : may have been examined
ACD : may be being examined
BCD : has been being examined
ABCD: may have been being examined
In these strings the different constructions are 'telescoped' into one another.
This means that combinations of the basic types A, B, etc form structures in
which the nonfinite verb of the first construction also functions as the auxiliary of the second, and so forth. For example, ABD (may haue been
examined) has the following structure, where have is shared by A and B, and
been is shared by B and D