The second sentence is correct. The word “boxes” is the subject of the sentence, so it must take the plural form of the vowel (in this case, “are”) to be in agreement. Benveniste, Emile. 1966. Relationships of the person in the verb. In the problems of general linguistics, 195-204. Coral Gables: University of Miami Press. In substantive sentences, adjectives do not correspond to the noun, although pronouns do. z.B. a szép könyveitekkel “with your beautiful books” (“szép”): the suffixes of the plural, the possessive “tone” and the uppercase /lowercase “with” are marked only on the noun. The Dikken, Marcel. 2001. “Pluringulars”, pronouns and bizarre correspondences.
The Linguistic Review 18: 19-41. Mithun, Marianne. 2003. Pronouns and chords: The information stand of Affixe pronominaire. Philological Society Transactions 101: 235-278. Here are some special cases for the subject-verb agreement in English: Enger, Hans-Olav. 2004. Scandinavian pancakes as a semantic convention. Nordic Journal of Linguistics 27: 5-34. Hahm, Hyun-Jong. 2006. Uniform or mixed consistency based on staff pronouns.
Presentation at the Midwest Slavic Conference. Changer, S. Mixed concordance, characteristic of the person and index/concorde distinction. Nat Lang Linguist Theory 29, 999-1031 (2011). doi.org/10.1007/s11049-011-9149-x “agreement” is the grammatical phenomenon, where the form of an element, such as the noun “horses”, imposes a second element in the sentence, such as the verb “gallop”, to appear in a certain form, that is, the “gallop” must correspond to the “horses” in number. Although convergence phenomena are among the best known and best studied aspects of grammar, there are some fundamental questions that have rarely been asked, let alone answered. This book develops a theory of correspondence processes in language and investigates why verbs are in agreement with subjects personally, adjectives in number and gender, but not the person, and nouns do not match at all. The explanation of these differences leads to a theory that can be applied to all parts of the language and to all languages. If, in the subject-verb concordance, the subject of the sentence is singular, the verb must also be singular. If the subject is plural, the verb must be plural.
Compared to English, Latin is an example of a very volatile language. The consequences of the agreement are: Samarin, William J. 1966. The Gbaya language: grammar, texts and vocabularies. University of California publications in linguistics, Bd. 44, Berkeley: University of California Press. Agreement or concord (abbreviated agr) occurs when a word changes shape, depending on the other words it refers to.  This is a case of inflection and normally implies that the value of a grammatical category (such as gender or person) “matches” between different words or parts of the sentence. King, Tracy H.
and Mary Dalrymple. 2004. Agreement ending and conjunction substantive. Journal of Linguistics 40: 69-104. . .