Help with a split infinitive needed here.....

[MrsFogi]

I would like to say "I had to swiftly gain the trust of both my new colleagues and their internal clients in various departments and roles around the company". Is this terrible?

I had swiftly to gain.

TBH, I would leave it as it is. The trouble with correcting many split infinitives is that the correction ends up so cumbersome it's not worth it.

Increasingly when I proofread stuff for clients I just leave split infinitives as they are for fear of creating something worse.

Personally I don't have a huge issue with split infinitives. The idea that you shouldn't split them stems, IIRC, from a time when it was decided that English should be as much like Latin as possible, and you don't split infinitives in Latin (because you can't, there not being two bits of an infinitive in Latin). There's no real reason for it (aware that I am hazarding my pedant status here). I find your construction far more elegant than "swiftly to go". Plenty of more pedantic pedants do care about split infinitives and would prefer the non-split construction, though.

to swiftly gain sounds much better... split infinitives are so 2009 doncha know?

Port's right, Fowler's says it came from not splitting infinitives in Latin (which you can't, however hard you try). They don't bother me that much but they may irritate some of the people reading your piece. And they feel a bit awkward - generations of people have had 'DO NOT SPLIT" drummed into them so it does make you pause when reading.

Why not use it as a sentence adverb?

'Swiftly, I had to gain...'

In fact 'Very swiftly,' makes it less odd for some reason.

I think I've been conditioned.

It does funny things to me.

It's like when someone says 'that's a whole (n)other story!'

Is it just me who wants to hit them?

Nothing wrong with it - doesn't cause ambiguity, no communicative problem. It's a stylistic issue, and some people will try to "correct" you. I don't think they've ever been terribly common in English, but the grammatical fuss is, I think, over a false analogy with Latin.

Can it be "I had to gain swiftly the trust of..."

I still think that all the alternatives to splitting it sound clunky.