Learn Hardcore Turkish: Akşam bahçede arkadaşlarla saklambaç oynamak çok keyifli. - Playing hide-and-seek with friends in the garden in the evening is very enjoyable.

The suffix -de marks the locative case (“in/at”). Here bahçe (garden) + -de → bahçede means in the garden. Because of Turkish vowel harmony, the vowel of the suffix matches the last vowel of the stem. bahçe has a front unrounded vowel e, so you use -de, not -da.

What does arkadaşlarla mean, and how is that form built?

Arkadaşlarla means with friends. First you make arkadaş (friend) plural with -lar → arkadaşlar (friends). Then you add the comitative/instrumental suffix -la → arkadaşlarla = “together/with friends.”

Why is saklambaç oynamak in the infinitive rather than a personal verb form?

In Turkish the infinitive (-mak/-mek) can function like a noun (a gerund). Here saklambaç oynamak literally means “to play hide-and-seek,” and that whole phrase acts as the subject of the sentence.

Why doesn’t saklambaç have an accusative suffix (like saklambaçı) in saklambaç oynamak?

Turkish only uses the accusative suffix -ı/-i/-u/-ü on definite or specific direct objects. Game names used generically (like hide-and-seek) stay unmarked. Adding an accusative here would sound odd.

What role does çok play in the sentence?

Çok means very and acts as an adverb intensifier modifying the adjective keyifli (“enjoyable/pleasant”).

Why does keyifli appear at the end, and why isn’t there a verb like “to be”?

Turkish is an SOV language, and predicates (including adjectives used as predicates) come last. There is no present-tense copula (“to be”) in Turkish, so an adjective alone can serve as the predicate:
[Subject phrase] + çok keyifli → “is very enjoyable.”

Why is akşam placed before bahçede, instead of after?

Adverbials in Turkish generally follow the order time → place → manner. So the time expression akşam (“in the evening”) comes before the place bahçede (“in the garden”).

What is the grammatical subject of this sentence?

The nominalized infinitive saklambaç oynamak (“playing hide-and-seek”) is the subject. Turkish often uses a whole infinitive phrase as the subject without any overt pronoun.

Can I replace keyifli with eğlenceli, and is there a nuance?

Yes, eğlenceli also means fun/enjoyable, but with a slightly different shade:

keyifli emphasizes a relaxed, pleasant feeling (“cozy/pleasant”).
eğlenceli emphasizes excitement or amusement.
Both are correct, but you’ll change the nuance slightly.

Why isn’t akşam suffixed with -de (i.e. akşamda) to mean “in the evening”?

Time words like sabah (morning), akşam (evening) often serve as adverbials without a case suffix. A bare akşam naturally means “in the evening.” You can add -da (akşamda) for emphasis or contrast, but it’s not required in most sentences.