Reduced relative clause

[Teo]

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1. The man who drives the car every day is my brother.
2. The man driving the car every day is my brother.
3. The boy who brings the milk has been ill.
4. The boy bringing the milk has been ill.
5. The boy who brings the milk every morning has been ill.
6. The boy bringing the milk every morning has been ill.
1 and 2 have the same meaning. But 3 doesn't correspond to 4. Why?
2 and 6 correspond to 1 and 5 respectively. But 5 is highly unnatural. Why?
Are there any rules?

[Alan]

Generally speaking, a reduced relative represents only a progressive form, hence the semantic non-correspondence of 4 to 3.

Regarding the other pairs, while it would be acceptable to say

The man who is driving the car every day...

,since the act of driving can be seen as extending over a long period, perhaps even most of the day,

The boy who is bringing the milk every morning...

is strange, since the delivering of milk (at least from the speaker's viewpoint) would normally be conceived of as a simple act of relatively short duration, not therefore naturally expressed via the progressive, hence the relative unacceptability of 6 (not 5: re-read my previous reply!).

Essentially, then, especially if you wish to avoid potentially difficult and possibly even culturally-based judgment calls relating to verbal aspect, the 'rule' is simple: do not attempt to use reduced clauses unless intending to represent a progressive meaning. While there may be any number of syntactically possible exceptions to this, not all will necessarily be inherently natural or contextually appropriate.