Digital Dollhouse

These two verbs are similar in that they both dictate the existence of something. Similar to desu です

iru - used for animate objects, such as humans and animals
aru - used for inanimate objects, such as furniture, books, etc

e.g. (examples from the nihongoUp lesson - broken up to help me understand method.

犬がいます
there is a dog - iru is made into imasu, for polite form, drop the ru - as it is a ru verb. 

ほんがあります
there is a book - aru again in formal form, this time the ru = ri, since it drops the u and adds the i.

-ru: it has an informal (dictionary) form e.g. taberu たべる (to eat) 

Current Positive Formal Forms:

Firstly, the formal form of verbs -masu (ます) this polite copula should be used in many situations. 
-ru: In the case of -ru （る） verbs, remove the -ru at the end and replace with -masu.
e.g. taberu たべる (to eat) becomes tabemasu (たべます) in formal form 

Current Negative Forms: (do not, am not, etc)

-ru: Formal negative, remove -ru（る) from the end, replace with -masen（ません）
-ru: Casual negative, same as above, but add -nai （ない)
e.g. taberu （たべる）becomes tabemasen（たべません) [formal] or tabenai (たべない) [casual]

Past Tense Positive:

-ru: Formal, replace -ru at end with -mashita (ました）
-ru: Casual, same as above, but use simply -ta (た)
e.g. taberu （たべる）becomes tabemashita (たべました)  [formal]  or tabeta たべた  [casual]

Past Tense Negative:

-ru: Formal, just add -deshita (でした）after -masen（ません） [the negative form above] 
-ru: Casual, replace the above -i (in -nai) from cas. neg. with -katta = nakatta （なかった）
e.g. tabemasen deshita （たべません　でした）[formal]  or   tabenakatta（たべなかった）[casual]