In the Lithuanian culture and language, the males and females within a family have surnames that are similar but end with different suffixes. You need to understand the system at least enough to convert a female surname into the family name.
In U.S. culture it is customary that the mother and father in a family have the same surname (last name) and that their children also share that same surname. When the daughter Jane Doe gets married to John Smith, she typically then becomes Jane Doe Smith or Jane Smith née Doe. With respect to Lithuanian surnames, there are some differences about our ancestors' Lithuanian culture that one needs to understand because those differences may have influenced how an ancestor's name is spelled in a U.S. record.
First of all, in the Lithuanian language and culture, all persons in the family have the same surname "stem" (or "root") but different suffixes. {Example: stem "Čepul-"}
Here are the "modern Lithuanian" suffix rules for surnames (These are based on information in "Introduction to Modern Lithuanian" by Dambriūnas, Klimas, and Schmalstieg (Reference 10)):
To form the married name for a female:
To form the "maiden name" for a female:
In most U.S. records of a Lithuanian immigrant ancestor, one will find the "Americanized" version of the ancestor's name. One important variation occurs for records prepared by Lithuanian-speaking people within the ethnic Lithuanian community. The primary occurrence of this is for the baptism, marriage, and funeral records prepared by Lithuanian-speaking priests in U.S. ethnic Lithuanian RC parishes. (And these situations get even more complicated because the priests didn't always spell names correctly and they had multiple ways of converting surnames into Latin for the Latin-language records of the RC parishes.) In the early years of those parishes, the people often spoke in Lithuanian, and so often used the Lithuanian forms of their names in their conversations even though they were using the Americanized form everywhere else. So when the priest wrote down their names in a parish record, he probably wrote down the Lithuanian form of the name even if they were using an Americanized form everywhere else.
The typical Lithuanian surname suffix endings -aitis, -avičius, and -evičius are all patronymic suffixes. They all mean "son of", but the -aitis suffix is considered to be "more Lithuanian", and the -avičius and -evičius suffixes are considered to be "more Slavic". Within my grandmother's family, some were using the family name "Antanaitis" and others were using the family name "Antanavičius".
In modern Lithuanian, the conventions for suffix endings of female surnames (as described above) are now standardized, but you may find other forms in records because there apparently were once regional differences within Lithuania. I have seen several instances where the maiden-name suffix ending is -ke instead of -te. In the 1910 U.S. census, my grandmother's name was entered as "Antanaicziuke" where I would have expected "Antanaitis" or "Antanaityte".
In present-day Lithuania, the conventions for spelling of surnames are now fairly well standardized, but in older records there are many more variations, and it is often difficult to know if the surname was simply misspelled in a particular record. My grandmother considered her family name to be "Antanaitis", but her brother considered his family name to be "Antonaitis". I use the spelling "Antanaitis" because this name is a patronymic one derived from the Lithuanian given name "Antanas".