The nominal agreement of the adjective “thank you” (obrigado), as an expression of gratitude, generates many doubts. In general, people do not know when to use “obrigado” or “obrigada”.
Nominal agreement is the agreement or harmony that must take place between the article, numeral, pronoun, and adjective with the noun. This happens with respect to gender (masculine and feminine) and number (singular and plural). This is the basic rule, however, there are some words or expressions that can create doubts when used. One of them is the adjective thank you (obrigado).
The rule of nominal agreement says that when thank you expresses a thank you, you must agree with the one speaking, i.e., the sender. See the example:
The teacher came into the classroom and praised the work of two students, Maria Isabel and Alberto. The students, of course, were very happy. So, they went to the teacher’s desk.
Maria Isabel spoke up: Thank you (obrigada), teacher.
Alberto spoke up: Thank you (obrigado), teacher.
Many people think that if we are thanking a woman, we should say ‘obrigada’. If the thank you is addressed to a man, then ‘obrigado’. However, this is not so. It is the gender of the person who is thanking that defines the agreement of this adjective. So, girls say thank you with (obrigada), while boys say thank you with (obrigado).
There is only one condition in which thanks (obrigado) should not be inflected, when it is a noun. In that case, it will remain in the masculine singular, regardless of who is speaking. Here it is important to remember that an easy way to detect the noun is to notice who accompanies it; article, pronoun, numeral, and adjective are grammatical classes that generally accompany the noun. See the example:
After being rescued, the emotional woman said, “My thanks (obrigado) to God and to all the firefighters who helped in the rescue.”
Important to notice that dictionaries do not present all the forms and that a word with a feminine form, in many cases, only appears in the masculine form (whether one likes this lexicographical machismo or not).
