I know this isn’t the first time a user ask this but I’ll try anyway.
I did a month subscription because I like the overall features of LingQ more than any other piece of language learning software, but the fact that different forms of the same word are treated as different words make it unusable for me. This is not a critic, I understand the work that is done and I appreciate it and all the content available almost for free. I’m writing this because I thinked of a way to add an option that will allow this behaviour, without need to broke anything.
I speak about japanese because this is the language I’m studying. I suppose you are using something like Mecab to break the words. And that every word is contained in a database table, identified with a unique number, like it is possible to see if you open a lesson and look at the html code.
So why don’t expand that database table with another column which contains something like this?
word | id | dictform_id
飲み | 000001 | 001
飲む | 000002 | 001
読み | 000003 | 002
読む | 000004 | 002
with the third column number being the same for the words forms which pertain to a single dictionary word?
So when a user makes a new lingq, he could choose “make a lingq for every other form of this word”. If this option is selected, the software looks at the dictionary_id of the word for which the user is making the new lingq, it searches all other words with the same dictionary_id and makes a lingq for those words too.
I think Mecab has a “deconjugation” option so starting from all different forms of the same word you could obtain the dictionary form and just tag every words with the same id when the dictionary form coincides.
This is only a suggestion, I don’t even know if LingQ works like this behind the scenes, but I know for sure this (apparently) simple feature would make more user subscriptions. Just talking about it in forums and the like, I know so many people that would subscribe but they don’t subscribe for this reason 
This is a shame… Just in my family my mother and my sister would subscribe too but are reluctant for this.
I know it is very difficult to expect this to be done, but I have a great esteem for Mr. Steve and all the staff on LingQ so I thinked to do at least an attempt to suggest this 
NB: This wouldn’t modify the way the words are counted or other Lingq features. It’s just like when you encounter those words separately, and I know for sure (or, at least in japanese) when a user encounter a new word he, if you give it the possibility to do so, would tag also all the other forms of that word. Only for now he has to tag them while he appens to encounter them while reading. So maybe he tag 読み as a new word, then he finally recognize it and maybe after three months he encounter a form of that word he never encountered before, so it is showed as a new words while it’s obvious he know it. It’s not a problem with a single word, but when you consider this multiplied for many words as would happen in a real situation, it defeats the purpose. Or, I repeat, at least for japanese where conjugations are so obvious it has little maning to treat them like words per sé.
I know the concept of word itself is blurry, so if one has two word’s forms it could be useful to treat them as different words (like in italian “vivere” and “vissuto”, or “leggendo” and “lessi”) because knowing one form of the word does not imply you automatically know the other forms. This could be also true in japanese, if only the words are broken the right way, but considering “飲みます” and “飲みません” are broken in a way that let you tag the stem “飲み” (so, a single lingq for both word’s forms), but then “飲む” is treated as a different word than “飲み”. So it would be good to tag them as different words as long as this behaviour is consistent, thing that is not in japanese, for obvious reasons… So you might as well settle things once and for all and permit users to tag all forms one time for all. If it is too difficult or impossible to do programmatically, just put a popup where you show all words which contain that kanji only (or that kanji compound only, if it is a compound) and let the user chose for which of those words to create a lingq. It requires only a search feature you can do it with regular expression just to restrict this to only words with the exact same kanji (or the exact same compound), so not inclusive. Please just take this into consideration
PS: maybe it is very difficult to do this and I’ve underrated the work needed for this to be done, I could only imagine all the works you have so I hope this doesn’t seem like a lack of respect, on the contrary it is only an affectionate concern
