Learn Hardcore Polish: Wczorajszy obiad był dobry.

wczorajszy obiad = yesterday’s dinner / the dinner from yesterday
wczoraj obiad by itself is ungrammatical; you’d need a verb:
- Wczoraj obiad był dobry. = Yesterday, the dinner was good.

So:

Wczorajszy obiad był dobry. focuses on the meal itself as “yesterday’s dinner”.
Wczoraj obiad był dobry. focuses more on the time (yesterday) when the dinner was good.

Both are correct, but they’re structured differently: adjective + noun vs. adverb of time + noun.

What is the grammatical gender of obiad, and how does it affect the rest of the sentence?

Obiad is masculine singular (inanimate). Because of that:

The adjective before it must be masculine singular: wczorajszy (not wczorajsza, wczorajsze)
The adjective after był must also be masculine singular: dobry (not dobra, dobre)
The past tense of być must be masculine singular: był (not była, było)

So you get:

Wczorajszy (masc.) obiad (masc.) był (masc.) dobry (masc.)

If the noun were feminine, e.g. kolacja (supper):

Wczorajsza kolacja była dobra.
- feminine forms: wczorajsza, była, dobra

Why is it dobry and not dobrym after był?

In Polish, when an adjective describes a noun (like obiad) and comes after być (to be), it normally stays in the nominative case, not instrumental:

Obiad był dobry. – nominative adjective (dobry)
Film był ciekawy. – nominative adjective (ciekawy)

You use instrumental mainly with nouns after być when describing what someone/something is (a role, profession, category):

On jest lekarzem. – lekarzem (instrumental, “a doctor”)
Ona była nauczycielką. – nauczycielką (instrumental, “a teacher”)

So here, dobry is a simple quality adjective, so it stays nominative: był dobry, not był dobrym.

What tense is był, and how is it formed?

Był is the past tense of być (to be), 3rd person singular, masculine.

Basic forms of być in the past:

masculine singular: był (On był – He was / It was [masc.])
feminine singular: była (Ona była – She was)
neuter singular: było (Ono było – It was [neut.])
non-masculine-personal plural: były (One były – They were [things, women, mixed but no male humans only])
masculine-personal plural: byli (Oni byli – They were [group with at least one man])

Because obiad is masculine singular, we use był:

Wczorajszy obiad był dobry. – Yesterday’s dinner was good.

Why isn’t there any word like “the” or “a” in this sentence?

Polish has no articles like English the, a, or an.

Wczorajszy obiad był dobry. can mean:
- The dinner yesterday was good.
- Yesterday’s dinner was good.

Whether it feels like “the” or “a” in English is decided only by context, not by a specific Polish word.

Can I change the word order, or is Wczorajszy obiad był dobry fixed?

Polish word order is relatively flexible. Wczorajszy obiad był dobry is the most neutral, but you can reorder things for emphasis:

Obiad wczorajszy był dobry. – possible, but sounds a bit literary/poetic or unusual.
Dobry był wczorajszy obiad. – emphasizes dobry (good), a bit more expressive.
Wczorajszy obiad dobry był. – also possible, somewhat stylistic or expressive.

For everyday neutral speech, Wczorajszy obiad był dobry is the best choice.

What case is obiad in, and why?

Obiad here is in the nominative singular:

It’s the subject of the sentence: What was good? – obiad (the dinner).
In Polish, the subject normally appears in the nominative.

So:

Wczorajszy obiad – both wczorajszy and obiad are nominative masculine singular.
The verb and the predicative adjective (był dobry) agree with that subject.

How do you pronounce wczorajszy and był?

Approximate pronunciation (in English-like spelling):

wczorajszy ≈ f-cho-RAI-shih
- wcz – like fch together:
  - w = English v/f (here it blends so it can sound close to f)
  - cz = ch in chocolate, but a bit harder
- o – like o in not
- raj – like English rye
- sz – like sh in shoe
- y – a hard i, like the vowel in roses (the second syllable), but tenser
był ≈ biw
- b – as in English
- y – same hard i sound as above
- ł – pronounced like English w

So był sounds close to bew or biw.

How would this sentence change in the plural, e.g., “Yesterday’s dinners were good”?

You need plural forms of the noun, adjective, and verb:

Wczorajsze obiady były dobre.
- wczorajsze – plural (non-virile) form of the adjective
- obiady – plural of obiad
- były – past tense, 3rd person plural (non-masculine-personal)
- dobre – plural adjective form

Everything still agrees in gender/number/case, but now in plural nominative.

Could we say Wczorajszy obiad jest dobry instead of był dobry?

You can, but the meaning changes:

Wczorajszy obiad był dobry. – Yesterday’s dinner was good (you’re judging it in the past, typical).
Wczorajszy obiad jest dobry. – literally Yesterday’s dinner is good.

The second one can make sense in contexts like:

You cooked the dinner yesterday, and you’re eating the leftovers today. You taste it now and say:
- Wczorajszy obiad jest dobry. – The dinner from yesterday is (still) good.

So był is the normal choice when you’re just talking about how the meal was when you ate it.

What’s the difference between obiad and kolacja? Does obiad mean lunch or dinner?

In Polish culture:

obiad – the main hot meal, traditionally eaten in the early or mid-afternoon. In English it can be translated as lunch or dinner, depending on context and local habits.
kolacja – the evening meal, usually lighter; closer to supper or (late) dinner.

So Wczorajszy obiad był dobry. is usually understood as:

Yesterday’s main meal (midday/afternoon) was good.
Not the late-evening meal; that would be:
Wczorajsza kolacja była dobra. – Yesterday’s supper was good.

How would I say “Yesterday’s dinner was very good” using this structure?

You simply add bardzo (very) before dobry:

Wczorajszy obiad był bardzo dobry. – Yesterday’s dinner was very good.

You can also intensify it with other adverbs:

Wczorajszy obiad był naprawdę dobry. – really good
Wczorajszy obiad był wyjątkowo dobry. – exceptionally good