Samson’s birth story deprecates Samson’s father, Manoah; this serves to highlight the identity of his real father: The angel of the LORD did more than announce Samson’s birth to Manoah’s wife.The grammar of the verb in verses 3 and 5 of Judges 13 is hard to interpret any other way. Grammar is your friend. And Professor Brettler's exegesis of the whole story makes good sense.
Was this one of the watchers who were left in heaven after the fall of the others? Was he sent by God on a special mission to impregnate a human woman to produce a semi-divine champion for Israel? That's very speculative, but I wouldn't rule it out. It fits the theology of the watchers in both the Enochic literature and the Hebrew Bible.
Deane Galbraith, call your office!
UPDATE: Reader Kenneth Greifer draws my attention to the similar construction in Deuteronomy 31:16, in which God tells Moses, "Behold you (shall be) lying down (i.e., dead) (הנך שׁכב) with your fathers and this people shall rise up and prostitute themselves after strange gods ..." Moses is clearly alive at the time.
The construction is similar, although "lying down" is a participle and "pregnant" (הרה) in Judges 13:5 is an adjective. But, yes, the latter could be read analogously with a future meaning drawn from the following imperfective verbal construction. So I agree that, although Professor Brettler's reading is possible and fits well with his overall exegesis of the passage, there is another way to interpret the verse: "For behold you (shall become) pregnant and you shall bear a son ..." In that case the angel need not have been directly involved with the pregnancy.
The coordination of the two verbal forms in v. 3 (והרית וילדת) vs. the disjunction of the adjective and verb in v. 5 (הרה וילדת) supports Brettler's interpretation, but this is not decisive.