Other contributors have said "In spanish what is an irregular verb?" is the same question as "What is a irregular Spanish verb?" If you believe that these are not asking the same thing and should be answered differently, click here
Irregular in the future tense: hacer (haré, etc.), poner (pondré, etc.), poder (podré, etc.).
Irregular in the past tense: decir (dije, etc.), dar (di, etc.), venir (vine, etc.).
Irregular...
Yes, because it is adding "en" to it. Written is the past participle of write write / wrote / written. They write everyday. They wrote yesterday. They have written everyday. Write is an irregular...
You never chose whether or not you are going to use a verb or irregular verb. Look at what you are trying to say, then pick the verb that fits the sentence. The verb may or may not be irregular.