usage - Is it ok to use the irregular past tense of a verb as it were a regular one?

Let's say I say / write catched instead of caught or buyed instead of bought, etc.

I know this is grammatically incorrect, but is it incorrect or perfectly fine to use it in every-day life ? English is not my first language so I am a little bit curious about this. Some people say it is correct and understandable, while others say it is incorrect and ridiculous.

Answer

You answered your own question when you said it was grammatically incorrect. It is usually understandable but is considered wrong.

That said, there are some verbs which have multiple forms. An interesting one is sneak which has both sneaked and snuck as past participles. Many people consider "snuck" to be incorrect. Indeed my spell check doesn't even recognize it. Yet for many others it is correct, if perhaps informal.

There used to be many more irregular verbs in English and over time a good number of them have become regularized. However, the most commonly used verbs are the most resistant to regularization because everyone already knows the irregular forms and they use them often.

As a non-native speaker you will probably be forgiven if you slip up and use a regularized form of an irregular verb. But, like my three-year-old son who also uses regular forms of plurals and past participles where fluent speakers would use an irregular form, you will be expected to correct them, especially if writing/speaking formally.