F

fewer/less

Fewer is used before a plural noun. Less is used before a singular (uncount) noun.

Yes: fewer people, fewer countries, less money, less time.
Yes: Fewer than one-fifth of the staff were involved, less than one-fifth of the cake was eaten.

But: 

• He weighed less than 40kg (where fewer would sound silly). 
• Fewer than half the people voted in favour (people only come in whole numbers). 
• Less than 50 per cent of people voted in favour (because it could be 48.5 per cent, not just 48 or 49 per cent).

field/frontline

Do not use. Instead, use project, country, or programme.

first-hand (adjective)

Hyphen, e.g. first-hand knowledge.

first hand (adverb)

No hyphen, e.g. at first hand.

Foreign, Commonwealth & Development Office (FCDO)

Spell out at first mention. Thereafter use abbreviation.

forever/for ever (adverb)

Not simply interchangeable. Forever means continually; for ever means for always.

If you can swap the word you want with for years, use for ever.

Yes: We’re forever discussing this.
Yes: We could discuss this for ever.

Forward slashes

No spaces either side i.e. either/or not either / or.

fractions

Best in words. Say half a million, two-thirds of people affected by leprosy, cases increased by one-fifth. 

Avoid ½, unless in a recipe.

fulfil, fulfils, fulfilled, fulfilling

In British English we use fulfil, fulfils, fulfilling, fulfilled (and fulfilment).

Do not use the American English double l in the present tense.

full stops (full points)

Insert a single space (not double) after a full stop: I’d like to. But I’m not sure I can.