Morning is a common English word, as you know.
Forenoon , on the other hand, is so rare that I'm not sure many native speakers of English will even recognize the word.
How rare is it?
To find out, I searched the Corpus of Contemporary American English (COCA) for both morning and forenoon .
Here are the results I got: Search term Number of results
-------------------------------------------
morning
128954 forenoon 16
That makes morning roughly eight thousand times more common than forenoon .
It's safe to say you should stick to morning and avoid forenoon entirely.
But wait!
Is it possible forenoon is only used in dialects of English other than US English?
To find out, I searched the Corpus of Global Web-Based English (GloWbE) , which contains samples of English from twenty countries.
And in none of those countries was it substantially more common than in the US; the numbers in every country were less than one occurrence per million words.
And the few results that I do find are mostly in fiction.
So yes, it is safe to say
: avoid forenoon .
Use morning instead.
ell.stackexchange.com/users/230/snailplane
It's not exactly obsolete, but forenoon is at the very least a "dated" usage...
Any native speakers would understand forenoon , but most people wouldn't use the term themselves, and they'd probably notice and classify it as a "minor mistake" if they heard a non-native speaker using it.
ell.stackexchange.com/users/126/FumbleFingers Reinstate Monica
More broadly than the other answers, forenoon is one of the time-of-day terms that has become much less commonly used over the last century or so, possibly as a result of the widespread use of artificial lighting.
In earlier times, it was common to see the day divided into much more specific segments, but now only the terms in bold are in common usage for describing the time of day:
dawn twilight (morning) sunrise morning forenoon midday afternoon evening sunset twilight (evening)
dusk night
As a fan of the Aubrey–Maturin series , set in the early 1800s, I'm quite familiar with the nautical term "forenoon watch", but I can't ever recall having seen "forenoon" used in a modern setting.
ell.stackexchange.com/users/2536/chrylis -cautiouslyoptimistic-
I grew up on a dairy farm in Dover, PA.
My family, as with most folks in the region, are influenced by PA Dutch (not Dutch as in from Holland but Dutch Anglicized Deutsch/German) dialects.
I'm not surprised it doesn't show up in Internet searches as I don't know that I've ever actually written the word but have used it commonly among my family and others from the region in conversation regularly.
ell.stackexchange.com/users/10630/Kate Rauhauser-Smith
I am a British, native speaker of English, living in Denmark.
I like the word 'forenoon' and sometimes use it, particularly in writing.
I do not regard it as archaic, but I may however be influenced in this by Danish, a language I speak every day and fluently.
In Danish, we distinguish between 'morgen' (morning) and 'formiddag' (forenoon).
We say 'God morgen!'
(Good morning!), but only up until about 09.30 or 10.00 hrs.
After that, we switch to 'Goddag!'
(Good day!).
I have always regarded 'forenoon' as a word much more used and favoured in Scotland than in England, and I admit that it is generally speaking far more used by older people than younger ones. '
Forenoon' has a nice ring to it, I feel. :-)
ell.stackexchange.com/users/25569/Peter T
I came across this word in some software code written by someone else. I knew what it meant (in Dutch we call it  voormiddag ), but I didn't know the word exists.  I've always heard/seen people refer to  morning  and  afternoon , never to  forenoon  and  afternoon . Is it a word people commonly use?
Is "forenoon" commonly used?
ell.stackexchange.com
<word-choice><time>
ell.stackexchange.com/questions/10900
ell.stackexchange.com/users/2343/Stijn
Although forenoon is not exactly obsolete, its usage is at the very least "dated" and much less used over the last century. It is so rare that many native speakers of English may not even  recognize  the word. It is much more used and favoured in Scotland than in England, and generally by older people. Forenoon is rare and dated usage, it is more likely to be used by older people.
Forenoon is rare and dated usage, it is more likely to be used by older people.