How can I write short poems or quatrains?
I am an English student in Iran. I am interested in writing 4-lined poems. Are there any rules I should follow? Are there any limitations in the line size?
Generally speaking, the "rules" of poetry are far more relaxed than the rules of prose. As far as structure goes (quatrains, stanzas, length of lines, etc.) the poet is allowed to do whatever he or she wants. If you are the composer, there is no limit to what you are "allowed" to do. Quatrain simply means "4 lines." These lines can be any length, rhyme and/or have a metered rhythm, or not.
Many people read poetry in the same way they might look at abstract or modern art. They assume that the thought behind it is very simplistic and often respond with something like, "Well, anyone could do that." Be careful. Though the rules of poetry are more relaxed, they still exist. I advise you to consider that although poetry often appears to be without rules, the basic rules of grammar are still present, they are just more lenient. This means, for example, that if you decide not to follow typical punctuation or capitalization rules, if you omit words or deviate from standard verb tense, or if you write without thought of standard English grammar, you should have a reason for doing so. A quick example is to consider that a line break can be used to create a pause, which might be visually more pleasing to you than a comma. This is acceptable.
It is very difficult to give advice about poetry because it varies so much and judging "good poetry" is as subjective as judging art. As you seek to develop yourself as a poet, I encourage you read other poets' work and write a lot. As you do, play with form, appearance, sound, structure, and rules. But keep in mind what you intend to get across with your poem. Do things purposefully, even if they only make sense to you at the time. And don't be afraid of several revisions to one poem. The best poets do not write a poem in a day.