This is not strictly about words, but about some constructions which have been discussed on various forums lately. It is not perfectly clear (to me, at least) whether these are changes from earlier situations, e.g., Pije's lessons, or clarifications or extrapolations from them. Nor is it clear that these are the final answers, but they are the standard for now (I think).
1. No 'pi' with 'nanpa' followed by a number. This does go against at least one example in Pije, but agrees with several early texts. The pattern is to bring ordinal numbers into line with cardinals: we do not use 'pi' with numbers which take more than one word: 'tu wan', 'luka luka' and the like. (Note, by the way, that 'luka' as "five," while regularly deprecated, is universally used.). Thus, just as "three men" is 'jan tu wan' and not 'jan pi tu wan', the third man is designated 'jan nanpa tu wan'. That is, 'nanpa' initiates a right grouped number.
2. Still on 'pi', prepositional phrases that are meant to modify nouns (are adjectival) require 'pi' before them. This is just a clarification of a rule in Pije that right grouped strings of two or more terms require 'pi' (though there are some early examples of violations in both directions even without prepositions). This is particularly important toward the end of the sentence, where the adjective PP might be confused with the terminal adverbial PP, the classic "I saw the man with the telescope" (mi lukin e jan (pi) kepeken ilo lukin).
But the rule applies in all positions: "The man with the telescope saw me too" (jan pi kepeken ilo lukin li lukin e mi kin).
3. Contrary to what Pije says, but in keeping with some examples, 'en' can occur in verb and DO position. But only for the mixed-lot case, not for the separable ("logical") "and". 'ni li loje en laso' is for something that is partly red and partly blue, but if there are more than one thing and some are red (totallty. for practical purposes, and others blue (ditto), the form is 'ni li loje li laso' and this is equivalent, as the first is not, to two separate sentences. Similarly, 'mi lukin e soweli en waso' is legal to describe a chimera of some sort (half beast, half fowl), but not two separate animals. each totally of its proper kind (mi lukin e soweli e waso').
4. Tying these together, compound modifiers -- which do take 'en' everywhere, whether mixed or logical -- require 'pi'. Thus, 'mi lukin e waso pi pimeja en walo'; again, a result of the general rule about modifiers of more than one word. This would apply equally to a particolored bird (or birds) and to several birds of one color each, though some different from others.
5. The issue of proper of various sorts involving 'toki' has come back into play, after lying dormant for several years with a satisfactory set of solutions -- though ones not natural to English (and, apparently at least Spanish, German, and French) speakers. The basic principle used earlier was that the DO of a verb had to be something that would also be referred to by that verb used as a noun (the moku e moku principle). Thus, the DO of 'toki' had to have a message ('toki' again) as its referent. So, the DO could not be a language or a topic talked about.
a. But the first of these, for which 'kepeken toki ...' was devised, came under attack, because a language is, of course, also a toki, so the standard argument falls through. Yet the 'toki e toki pona' expressions always feels like a mere carry over of L1 habits and does not have a separate justification within tp. One can, of course, be found easily by taking the expression as short for (a antidittophatic collapse of) 'toke e toki pi toki pona', which collapse seems likely to have occurred, given the opportunity. The compromise, which avoids the supposed unnatural 'kepeken' construction and also puts the the language in a modifier role, is to make the verb 'toki pi toki pona', for which there are some early examples. On the whole, my preference remains with 'kepeken' and the verb modifier as a legitimate alternative (but see below) and shun the DO form. But the question is again open.
b. The "about" question is not so clearly opened as that of the language used. It is opened, if at all, by the apparent expansion of the DO of 'toki', even though the cases are not really parallel -- the topic is not a toki, even if the general term for "topic" were 'toki' (which I am not sure it is; it seems more like 'sona' to me -- or 'pilin' or 'nasin'). On the other hand 'e ijo [(pi) topic]' does seem artificial, though literally accurate (it seems a Lojbanic solution, if you will). The alternatives proposed to the 'ijo' construction are to use the topic as a modifier to 'toki' or to invent a preposition with the meaning "about". The first of these introduces another ambiguity: both with the above "in language" construction (for languages might be topics of discussion, as tp often is) and with the usual adverbs of manner: is 'toki wawa' shouting or talking quietly about force? On the other hand, no preposition leaps to mind as the right one for "about." 'lon' has some history in other languages: "on", for example, but that is clearly just L1 relexing and the same applies to creating new prepositions like 'sike' (though the process of making a word a preposition is not a problem per se). On the whole, this issue seems the least in need of revision, but the possibility has arisen,