だTo be, Is

だ and です both function like the English 'is.' The main difference is politeness: だ is casual and used in plain speech, while です is polite. だ can't be used after い-adjectives, but です can, acting as a general politeness marker. You may still see だ in set expressions like ～だと思います, even in polite sentences.

The main difference between だ and である is formality. だ is casual and common in conversation, while である is more formal and mainly used in writing. Both だ and である follow nouns or noun phrases, but である has a more literary tone. である slight differs from the verb ある (to exist) in that である is a fixed connector.

だ conveys a casual, informal, or blunt tone, except when required by certain grammatical patterns. In contrast, でございます is a highly polite form, primarily used in business settings or when addressing someone of higher social status such as a boss or customer.

だ follows nouns and な-adjectives, with a meaning similar to the English 'is.' For instance, これはペンだ translates to 'This is a pen.' On the other hand, じゃない (a contraction of ではない) is used to create the negative of such expressions. For example, これはペンじゃない translates to 'This isn't a pen.'

だった・でした and だ are all used to assert or identify something. The difference among them is in formality and tense. だ is casual and present tense (similar to the English 'is'/'am'), while だった・でした is past tense ('was'), with だった being casual (the past tense of だ) and でした being polite (the past tense of です).

がある indicates the existence or possession of something. For example, ペンがある means 'There is a pen (on the table, etc.)' or 'I have a pen.' On the other hand, だ expresses assertion or determination similar to 'is/are' in English. In contrast to the がある example, ペンだ would mean 'It (=the thing we are looking at) is a pen.'

よ is a common sentence-ending particle used to add emphasis, often indicating the speaker is sharing new information (e.g., 面白いよ = 'It's interesting, I'm telling you!'). だ is a grammatical form expressing assertion, similar to 'is' or 'am.' These two are often combined, especially in casual or masculine speech (元気だよ = 'I'm feeling good!'). In softer or more feminine speech, the だ is frequently omitted (e.g., 元気よ).