Learn Hardcore Russian: Неделя была длинной и сложной.

Russian has no articles (no “the” or “a”). Definiteness is shown by context or demonstratives like эта (“this”) or та (“that”). So Неделя была длинной и сложной means “The week was long and difficult” without any separate article.

Why is была used here? I thought “to be” is often dropped in Russian.

In the present tense, the verb быть (“to be”) is indeed omitted: you’d say Неделя длинная и сложная (“The week is long and difficult”). But in the past and future tenses you must include the copula. была is the past‐tense, feminine‐singular form of быть, agreeing with неделя.

Why is it была and not был or было?

Past‐tense forms of быть agree in gender and number with the subject. неделя is feminine singular, so you use была. If the subject were masculine you’d use был, neuter было, plural были.

Why are длинной and сложной in these forms instead of длинная and сложная?

After the verb быть in past and future tenses, predicate adjectives take the instrumental case. Feminine singular adjectives of the -ая type change -ая to -ой, so длинная → длинной, сложная → сложной.

Can I say Неделя была длинная и сложная with nominative adjectives?

That’s colloquial and less correct in standard Russian. After была, the normative form is instrumental. Nominative adjectives like длинная and сложная are used only in present‐tense predicates without a verb.

Why do both adjectives end in -ой and not -ей?

First‐declension adjectives (ending in -ая) have two instrumental endings: -ой after hard stems and -ей after soft stems. длинный and сложный are hard‐stem adjectives, so they take -ой.

What’s the difference between длинной and длиной? They look almost the same.

длинной (double н) is the instrumental of the adjective длинный meaning “long.”
длиной (single н) is the instrumental of the noun длина meaning “length.”
E.g., длиной в километр = “with a length of one kilometer.”

Do I need a comma between длинной and сложной?

No. When two adjectives are joined by the coordinating conjunction и (“and”), you do not use a comma. If и were omitted, you would separate them with a comma: длинной, сложной.

Could I change the word order to emphasize an adjective?

Yes. Russian word order is flexible. The neutral order is Неделя была длинной и сложной. For emphasis you can say Длинной и сложной была неделя, which sounds more poetic or stressed.