I see no difference between "he only ate sweets" and "he ate sweets only". I have only encountered " que" after the past participle before the object

The difference is this:

He only ate sweets: he didn't do anything else but eat them.

he ate sweets only: he didn'eat anything else but sweets. 

-- Chris. 

Yes, in this particular example but what about this one:

Elle n'a qu'embrassé Alex. -- She only kissed Alex. 

Elle n'a embrassé qu'Alex. -- She kissed only Alex. 

My native speaking French friends question whether "Elle n'a qu'embrassé Alex" is good or idiomatic French to begin with. I speak fluently and frequently and have never encountered this construction. 

Parenthetically, to say "She only kissed Alex" isn't even very good English and is at best used only out loud - not in written form - as the phrase is subject to interpretation, i.e. it's not clear. The position of ONLY in English is problemmatic to begin with consider these placement versions.

Only she wanted to kiss Alex. (She was the only one who wanted to kiss Alex)

She only wanted to kiss Alex. (She wanted to kiss Alex - no big deal deal, OK?)

She wanted only to kiss Alex. (She wanted to kiss Alex and nothing more.)

She wanted to only kiss Alex. (She wanted to kiss Alex and nothing more - a more awkward version of the preceding line.)

She wanted to kiss only Alex. (The sole person she wanted to kiss was Alex - Alex and only Alex)

She wanted to kiss Alex only. (She wanted to kiss Alex and no one else - pretty much the same as the preceding placement).

To say "she only kissed Alex" where that phrasing means "she merely kissed Alex (and nothing else)", one might say "elle a embrassé Alex, et rien de plus."

I know this isn't an English forum, but the above might underline the problems both languages have with placement of exclusionary QUE and ONLY. Personally, I think you're better off trying to avoid both, since both are subject to confusion.

max

I think the example now given does not differentiate between either meaning, that is to say either example can be used to express either meaning depending on intonation when speaking. If I meant that it was only Alex whom she kissed I would use "seulement". If I wished to say that she kissed and nothing else I would say " Elle n'a fait qu' embrasser..."

So far I have found no instance other than this lesson using the construction " n'a que.." plus past participle, rather than " n'a past participle que"

I note that Max shares a similar view. Please would you cite an example in literature where the " n'a que" construction is used.

Vous avez fait mouche!

https://www.babelio.com/livres/Roy-Traces/236987
Je n'ai rien fait d'important aujourd'hui, je n'ai que vécu.
La sérénité à inventer, d'un geste à l'autre, l'enchaînement des actes... vers quoi ?
La vie, simplement. La vie allant vers la vie et se justifiant soi-même. La vie, fin de la vie. 
Sans but, mordre les choses, les minutes, les espaces. 

Very drôle but vecu here is used as an adjective not as a past participle.

How can it be an adjective after avoir? Of course it's a past participle.

You are right. Thank you for the citation. I do, though, find the expression strange. Life, however, is too short to spend time discussing such niceties. I shall continue to avoid such usage, despite the lesson until I have researched it further. I do though appreciate your comments.