Sometimes it is necessary to separate two different ideas, when -o is something more abstract and -aĵo something more concrete.
La libro vekis mian intereson.
Mi aĉetis kelkajn interesaĵojn.
In other cases it makes no difference. According to PIV (vortaro.net), for example, one of the definitions of havo is havaĵo.
The roots themselves have a word class. Knowing this helps in order to understand the pattern.
Hav- is a verbroot, while nov- is an adjective root. After a while you will get a feeling for this but you can always check it in PIV (vortaro.net). When you search for any word the first word on the page will include the major root and the ending which corresponds to its inherent word class.
As previously covered by others, when -aĵ is added to noun roots it can mean ”made of”:
- pasto = paste/dough
- pastaĵo = pasta
I have also encountered it in slang meaning a piece or concrete thing of something more abstract.
Mi ne komprenas tiujn matematikaĵojn en la raporto.
I don’t understand that math stuff in the report.
Verbroots usually follow:
- havo = the act of having
- vido = the act of seeing
- aŭdo = the act of hearing
You are probably more used to see that type of words ending in -ado. Note that -ado suggests a long-lasting or repeated action. It is left out for single or short actions.
Unu vido tauĝas pli ol dek aŭdoj.
Je la unua vido la tasko ŝajnis facila.
Now, adding aĵo to the verbroots we get:
- havaĵo = something possesed; a possesion
- vidaĵo = something seen; an image
and so on
For some of the roots either -o or -aĵo can be added to communicate the same thing. As mentioned above that is the case with hav-.
For many roots that does not work. I recommend always distinguishing between the act and the object to avoid confusion.
Aĵo combined with adjective roots makes things with those qualities, similar to how -ul is used to refer to people with certain qualities.
- novaĵo = something new; a piece of news, novelty
- malfacilaĵo = something difficult; a difficulty
Novo and malfacilo are not used much in my experience, and they might as well refer to the quality or state the adjective is describing. I recommend using -ec and -aĵ, again, to avoid confusion.
Another reason to watch out are quantity words such as lenght, weight, speed and so on. They are roots (usually adjective) combined with -o.
- alto - height
- alteco - highness
- altaĵo - something high
If you want to say for example ”a weight”, ”large stuff” or ”something long” you need to use -aĵ.