'ei/en/et' = 'a/an'. This is the indefinite article for any noun. All nouns in Norwegian have a grammatical gender, and 'ei/en/et' corresponds to a specific gender. These cannot be interchanged.
'ei' is for feminine nouns, 'en' is for masculine and 'et' is for neuter nouns. The gender of a nouns has to be memorized when you learn the word. 'måltid' is a neuter nouns, so you would always use the indefinite article for neuter nouns: 'et'.
To express the definiteness of a noun, you add a suffix to the noun. For neuter nouns this is '-et'.
'et måltid' = 'a meal'
'måltidet' = 'the meal'
Not trying to be that one guy, but I checked the tips and notes in the top right in basics 2, and it says that ei=feminine, et=neuter, but en=masculine or feminine? According to that, most Norwegian speakers use ei for feminine, but apparently it's also possible to use en for feminine. Like in basics 1, where it says "click the translation of 'a woman'" and the correct answer was "en kvinne"
'en' is used for masculine nouns, but it's common to inflect many feminine words as if they were masculine, so you can say either 'ei kvinne' or 'en kvinne'. Whether or not a feminine noun is treated as a masculine one often depends on its usage. 'kvinne' is usually treated as masculine, whilst 'jente' is feminine.
In a lot of European languages, it is common to put the verb first if asking a question. However, it depends on the specific language of whether or not exceptions are tolerated. EX, in German, Du kommst? is an acceptable version of Kommst du? You just have to make sure to raise your tone slightly to indicate questioning.