My brain is like a sieve.
Be honest, how often do you have to peek at words because you forget things?
The amount of times I see a a piece of vocab or a verb that I've seen a lot before but I still can't remember what it means is really annoying.
People that have been here longer than me, does your memory improve naturally over time or do you manually go through and memorise everything?
I'm hoping I'm not the only one like this :P
The short answer is yes.
Peeking is not bad, it is called a lesson and not an exam for a reason (there is the option to test for a skill, and then it is really an exam, and peeking is deactivated). You are doing the lesson in order to learn new things you do not know or refresh things you saw before but don't remember. After each lesson you remember more words, and after the second or the third, or fourth... time you see a word (that you have to peek) you eventually realize that you don't have to peek anymore, and this 20 letter word that when you saw for the first time you thought "I will never be able to remember how to spell this" it becomes trivial to spell. Remember that it is also important to do lesson reviews and practice, not only new lessons.
You're definitely not the only one. Give it time, though. I peek a lot in German, but I almost never have to in French, now. If you see the words enough times, you'll remember them eventually.
If there's a particular word that's really giving you a lot of trouble, try writing it down on a sticky note and putting it somewhere visible for a while.
Same problem, and I've done some digging around; a lot of people say that using the word in real life in context makes a word stick in your brain a lot better then rehearsing it over and over on a screen or flashcards (which is why they also say to find someone who you can talk to in your target language because you have to -use- words, not just remember them). Hope you find something that works for you, good luck!~
I remember most words that are really used a lot (like have, do, write, eat, drink, pronouns, prepositions, man, woman, etc.) without any effort. They are repeated all the time.
I forget the words that are only used in one unit, like some animal names that are not repeated later. I remember taking a lesson on marine life on another site. I forgot the names of all those fishes and animals right away because I didn't use them anywhere else. I don't care about such words. If I ever want to read a book about marine life or talk about fish to somebody, I'll use a dictionary and I'll most likely remember some most common words.
If you struggle with common words, just practice more. Maybe you should only take one new lesson a day and have 5-10 practice sessions for the previous lessons. You can even have "practice-only" days from time to time to make the old things stick in your memory.
I don't know if it is much comfort to you, but some units on Duolingo are really ineffective, so in many cases it is not the "fault" of your brain if you can't remember the words. The most awful are Adverbs (I can confirm that for French and German). It is extremely hard to learn about 50 adverbs without much context. I felt my brain was like a sieve, too, when I did those lessons. I picked up most French adverbs later while reading books with a dictionary, so I guess I should not have struggled too much with those Duolingo units.
I too struggled with adverbs (in Spanish), and still find it hard to retain them.
However, I do find flash-cards essential, and use them to keep up with the vocabulary I'm learning on Duolingo. It seems to me that the Duolingo lessons aren't enough in themselves to keep my vocabulary up-to-date, and flash-cards better for that, probably because you're exposed to far few words in a given time period than you would be riffling through flash-cards.
That said, Duolingo does work wonderfully in teaching new vocabulary and giving practice in using words in sentences, as opposed to the "what's the Spanish for 'almost'?" type of revision that my flash-cards give.
(I'm sure I'd benefit hugely from talking with a Spanish speaker, but there isn't the opportunity locally without paying for lessons, which I can't afford.)
Incidentally, I'm 53 years old, so memory for me is a bit more of a challenge than it used to be. That's one of the reasons I'm learning a language, to try to keep my memory in shape.