Learn Russian: Гроссмейстер вынужден думать о защите, чтобы не получить мат. - The grandmaster is forced to think about defense in order not to get checkmate.

Вынужден acts as a modal predicate meaning "forced to" or "compelled to." Because it is a short-form adjective, it must agree in gender and number with the subject it refers to. Since гроссмейстер is a masculine singular noun, we use the masculine form вынужден. If the subject were feminine, it would be вынуждена, and if plural, вынуждены. It is followed directly by an infinitive (here, думать).

How would this sentence change if the grandmaster "was forced" or "will be forced" to think about defense?

Because вынужден is a short adjective and not a verb, it does not have its own past or future tense forms. To change the tense, you must add the appropriate form of the helping verb быть (to be). For the past tense, you would say Гроссмейстер был вынужден думать... and for the future, Гроссмейстер будет вынужден думать....

Why is чтобы followed by the infinitive получить instead of a past-tense verb?

When the subject of the main clause and the subject of the чтобы (in order to) clause are the exact same person, Russian requires чтобы + infinitive. Since the grandmaster is both the person thinking and the person who wants to avoid checkmate, we use the infinitive получить. You only use чтобы + a past-tense verb when the subjects of the two clauses are different (e.g., "I want YOU to do it").

Why do we use the perfective verb получить after the negative не? Aren't negated verbs usually imperfective?