Tenses... Oh Dear

So I've been working on my Japanese studies, though at a slower pace because of midterms/finals this month/grad school admissions/other fun stuff.

Today I learned that stuff like です does not mean "It is", but rather is a tense modifier, making a sentence present positive. Growing up, I was always good at reading and writing, but grammar was something I struggled with. I could use it correctly the vast majority of the time from reading so much, but I couldn't tell you what a participle was. I could just tell if a sentence was incorrect and fix it, but not WHY it was incorrect. Apparently I'm going to have to relearn some grammar terms.

です- makes a sentence present positive
でした- makes a sentence past tense
じゃありません- makes a sentence negative (present negative?)
じゃありませんでした- makes a sentence past tense negative

Also, yay particles! o_o

Vocab Words:
じてんしゃ- bicycle
おてあらい- restroom (polite)
みず- water
しお- salt
でんしゃ- train
ちかてつ- subway
せんせい- teacher
ともだち- friend
こどま- child
びょういん- hospital (struggling to pronounce this one, byo-u-i-en?)
くすり- medicine
しごと- work/job
きょうかしょ- textbook
はる- spring
あき- autumn
ふゆ- winter
なつ- summer
ゆき- snow
あめ- rain
こうえん- park
ねこ- cat
つくえ- desk
りんご- apple
さとう- sugar
いぬ- dog
しんぶん- newspaper
しゃしん- picture
じしょ- dictionary
はこ- box

一人- alone, one person