Why are these sentences correct or incorrect?
Can you please explain why the letter "ی" is attached to the words "درختان" and "خیابان" in these sentences? It seems that there is a grammatical rule dictating their presence which I don't yet understand.

.کنار خانه درختانی هستند
.جلوی خانه خیابانی نیست

Perhaps it would be helpful to explain why the following sentences are incorrect:

.کنار خانه درختان هستند
.جلوی خانه خیابان نیست

Thank you for the help.
((خیلی ممنون برای کمکتون و وقتی که شما ته من گذاشتین

Question

Why are these sentences correct or incorrect?
Can you please explain why the letter "ی" is attached to the words "درختان" and "خیابان" in these sentences? It seems that there is a grammatical rule dictating their presence which I don't yet understand.

.کنار خانه درختانی هستند
.جلوی خانه خیابانی نیست

Perhaps it would be helpful to explain why the following sentences are incorrect:

.کنار خانه درختان هستند
.جلوی خانه خیابان نیست

Thank you for the help.
((خیلی ممنون برای کمکتون و وقتی که شما ته من گذاشتین

Answer

The enclitic «-i/ـی» is the indefinite article (singular and plural). Alternatively, you may use the articles «chand/چند» (plural), and «yek/یک» (singluar). The lack of an article indicates the noun is definite, e.g. «mardi/مردی» =  a man; «mard/مرد» = the man.

Answer

maybe you say it in English like: there are trees!
but in Persian you need to say something like: there are "some" trees! / there are "a few" trees. (with some preposition)
this "ی" performs like this like "some" and you need to use it.

and

you say there is no street in front of the home
but in Persian you need to say something like: there is not "a" street in front of the home.
this "ی" performs like "a".