Learn Hardcore Russian: В понедельник я обычно читаю новости и планирую неделю. - On Monday I usually read the news and plan the week.

Because in Russian, time expressions using в (to mean “on” for days of the week) take the accusative case. So в понедельник means “on Monday.” The accusative singular of понедельник is понедельник (unchanged).

What is the difference between в понедельник and по понедельникам?

в понедельник refers to a specific Monday (for example, this coming Monday).
по понедельникам means “on Mondays” in general, indicating a habitual action every Monday.
If you said по понедельникам, it would mean “I usually read the news and plan the week every Monday” in a general sense.

Why is обычно placed between the subject and the verb instead of at the beginning of the sentence?

In Russian, adverbs of frequency like обычно can appear before or after the subject (and sometimes even after the verb) without changing the core meaning. Placing обычно after the subject я emphasizes the personal habit. You could also say Обычно я читаю новости…, which puts slightly more focus on the routine aspect.

Why are читаю and планирую in the imperfective aspect here?

Both verbs describe habitual actions (something you do regularly), so the imperfective aspect is used:

читаю (imperfective of читать) conveys ongoing or repeated action “I read.”
планирую (imperfective of планировать) means “I plan.”
If you used perfective forms (прочитаю/ запланирую), you’d imply a single completed action, not a habitual one.

What case is новости, and why is it plural?

новости is in the accusative plural. For inanimate nouns, the accusative plural form is identical to the nominative plural (новости → новости).
Russian always treats “news” as a plural noun, unlike English, where “news” is an uncountable singular.

Why is there no preposition before неделю, and what case is it?

неделю is the direct object of the verb планирую, so it takes the accusative case (singular).
No preposition is needed because планировать directly governs a noun in the accusative to express “planning something”: “to plan the week.”

How would you modify the sentence if you wanted to say “I usually plan my week” instead of just “the week”?

You could add a possessive pronoun:
В понедельник я обычно читаю новости и планирую свою неделю.
Here свою (accusative feminine singular) specifies “my week.”

Can the word order change in this sentence for emphasis?

Yes, Russian word order is flexible. For example:

Я обычно в понедельник читаю новости… (emphasizes time)
В понедельник читаю я обычно новости… (emphatic, highlights the subject)
But the original order is neutral and most common.