Cambridge Grammar of English Hardback with cd-rom a comprehensive Guide

Conditional if

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Conditional if, where considered obligatory in formal speech and especially in

formal writing, may occasionally be absent from initial position:

They turn up at any point, just let me know.

(understood: If they turn up …)

[host at table] 

You want anything else, just help yourself. 

Fixed expressions

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Many everyday fixed expressions are prone to ellipsis of initial elements, since

these can be assumed to be known by all participants:

[said just as rain starts to fall]

Good thing I remembered the umbrella.

(understood: It is a/It was a good thing …)

Oh, 

good job I’ve left a little hole, then.

(understood: It’s a good job …)

Other common expressions that occur with initial ellipsis include:

(There’s) no point in …

(It’s) not worth …

(It would be) best if you …

(I’ll) see you later/tomorrow/soon …

(I’ll) be seeing you …

(You) never know, …