Hello, Greetings! of the day. As mentioned in project description, your requirement is to display custom fields on product page as follows:
1) There will be checkbox field with label "TAK" and the question, "Do you need literki?".
2) If this field is checked, it will display other field. The other field will be quantity field, with minimum value as 1 and maximum value as 5, label Quantity, number type integer. When value is added in this field, the product price will change, using formula, number entered, multiplied by two.
3) When value is put in Quantity field, there will be required textfield displayed, with label Sentence, maximum number of letters, same as the value of Quantity field. The updated price and values of these fields will need to be displayed on checkout page, and saved with order, when order is placed.
To implement this functionality of adding custom fields on product page, display custom fields on checkout page, save custom fields with order data, the code will need to be written, using Woocommerce hooks and filters. I have much experience in doing customizations in projects, using Wordpress, Woocommerce. Your work will be done, at the earliest, according to requirements.
Thanks.