All of the words and phrases belong in one or more lessons. Lessons are the heart of the way Lingopolo works. You should firstly make sure that you have read the FAQ "What is a lesson?" which will give you a general view of what lessons on Lingopolo are.
Lessons for a particular word or phrase are set on the "Lessons" tab. For example, the image below shows the word "chicken" being added on the "Birds" themed lesson:
Phrases and Words have different rules as to what lessons they should belong to.
Phrases
Phrases are easy to assign: in general, they belong only in the "Recording Type" lesson called "Phrases". Even if there is a phrase like "the dog", or "dogs are pets", because it is a phrase, it should not be put in the "Animals" lesson. The "Examples" lessons will automatically pick up all the phrases which contain items of a particular type. For example, the Animals Examples lesson contains all the animals in the Animals lesson, together with all the example sentences which use those animals, so phrases like "the dog" or "dogs are pets" will automatically get included for this Examples lesson.
Only words go in lessons other than the "Recording Type" lesson. There are occassional exceptions, where a phrase is really behaving as a single word, but the general rule is that phrases are only in the Phrases lesson and no others.
Words
Words require a lot more attention.
A word firstly should go in the Recording Type lesson Words.
A word will also belong in one or more Part of speech lessons. For example, all nouns (e.g. dog, house, car, ants) belong in the nouns lesson. All verb parts will go at least in the Verbs (all parts) lesson. Verb infinitives, for example should also go in the Verbs (infinitives) lesson. Check out all the parts-of-speech lessons on the Online Lessons page. Every word must be a part of speech of some type. It is sometimes not easy to tell, but in that case Google can of course help to decide with a query like "What part of speech is the word...?".
The other main group of lessons is the Themed lessons, lessons like Animals, Body parts, Circus etc. Again, take a look at the Online Lessons page to see a complete list of Themed lessons. Note that a single word can often appear in more than one Themed lesson. For example, chicken would appear in both the Birds lesson and the Animals lesson (as would all the birds). As another example, the word book belongs reasonably in the three lessons Bedroom, Living Room, and Office.
If you really cannot see a Themed lesson where the word belongs, the word should be put in the Miscellaneous themed lesson.
Examples lessons
When you look at the Online Lessons page, you will see many "Examples lessons", e.g. the Animals Examples lesson. These lessons are automatically generated from the related words lesson. For example, all of the example sentences from the animals in the Animals lesson will appear in the Animals Examples lesson.
Why lesson assignment matters
Only words which appear in lessons get practised and drilled by students. Therefore, there is no point doing lots of high quality recordings and careful administration if the words are not correctly assigned to the relevant lessons; students will simply never end up practising the words. If you think of it like books in a library, the lesson assignment is the categorisation of a book; a book in a library without a reference will have trouble being found.