With respect to your own name, you can definitely refer to both of your parents' last names collectively as your last name or your surname. It is also used collectively in the same way a single last name would be: for example, even in English-language bookstores, the books of Mario Vargas Llosa are alphabetized under "V".
Formally, a last name made ...
You can refer to the apellido paterno as the paternal surname and the apellido materno as the maternal surname. However, you cannot assume that all or even most English speakers will know what these terms mean without an explanation.
If you want to refer to both surnames together, simply using "last name" is probably not a good choice, as it's very ...
If you just want to use both names without having to explain the different system, the simplest answer is to hyphenate them into a single name.
Some married women, especially those with an established career, choose to hyphenate their maiden name and husband’s name as a balance of maintaining continuity vs social convention. (Note that the children in these ...
The phrase "as soon as January" is correct as it is. "As soon as at January" isn't idiomatic.
Here is a dictionary that shows "as soon as" as a conjunction when it is followed by a clause, and as a preposition when it is followed by a noun.
Wiktionary "as soon as"
I came as soon as I could.
How many girls whose ages are less than or equal to 18 do you have?
You could say:
How many girls do you have who are aged 18 or under?
How many girls do you have who are under 19 years of age?
Almost right, but each time the thread goes between the two pieces of fabric or skin that we wish to join, it's a stitch. We use the number of stitches as an indicator of the severity of the wound. To describe your photograph, you would say
He had to have 13 stitches in his head.
In my passport issued while the UK was still a member of the EU the first page has Surname/Nom (1). In the key on the next page to the numbers (1) in Spanish is Apellidos so the EU expected to find both apellidos there. You do not state your nationality but if you need this information for official purposes this may help.