を is the particle of the accusative case. That means that it shows the object that is having something done to it. You can check it by asking question "who" or "what" after the verb. For example "Mari invited who?" - "Mari invited Jon". But you can't use を in the second sentence, cause chair here is not the target of the action. Cause "sit" is an intransitive verb, that means that it doesn't allow an object of the action. Here you should use the particle of place/direction/etc. that is に. So it will be "マリがいすにすわった". Here いす - chair, すわった - sat (すわる - to sit). Also though it's possible to use が here, particle は would me more suitable. Cause が makes accent on the word it attached to, so you want to use it here only if you want to say that it is Mari sitting there, not some else.

Ciao Malene, >Jon o Marî ga sasotta -> Mari invited Jon. >Mari ga on a chair o sat (?) -> "Mari sat on a chair" was it what you wanted to translate? "Mari ga isu (chair) ni suwatta" should be the translation. Instead of "ga", wa can be used and gramatically correct. But it has a slightly different meaning in the context; with "ga" it implies that there were somebody and "ga" gives stress that it was Mari who sat and not others.