Learn Hardcore Japanese: saikin watasi ha amai mono bakari tabete hutorimasita. - Recently I gained weight because I eat only sweets.

最近 means “lately” or “recently.” Placing it at the very start sets the time frame for the whole statement, just like beginning an English sentence with “Lately,” to let the listener know when the action took place.

Why is 私 marked with は, even though the subject is probably understood?

Using 私 with the topic particle は:

Emphasizes that you (the speaker) are the one doing the eating and gaining weight.
Clarifies the topic, especially in situations where you want to contrast yourself with others or simply make it explicit.

Why is 甘い物 used instead of just 甘い?

甘い is an adjective meaning “sweet,” but you need a noun to say “sweet things” or “sweets.” Adding 物 (もの, “thing”) turns it into a noun phrase:

甘い物 = “sweet things” (i.e., sweets, desserts).

What does ばかり indicate in 甘い物ばかり?

After a noun, ばかり means “nothing but” or “just.”

甘い物ばかり literally means “nothing but sweet things,” implying you ate only sweets and no other kind of food.

Why is 食べて in the て-form before 太りました?

The て-form here links two verbs to show cause and effect:

食べて (ate) → 太りました (gained weight).
It functions like “I ate (sweets), and as a result, I got fat.” This chaining is very common in Japanese for sequential or causal actions.

Why is 太りました in the past tense rather than the present?

太りました is the past polite form of 太る (“to get fat”). The past tense indicates that the weight gain has already happened, so you’re reporting a completed change of state: “I have gained weight.”

Could I use から to explicitly show the reason, like “because I ate too much sweets”?

Yes. For example:
最近甘い物ばかり食べすぎたから太りました。

Here から (“because”) explicitly marks the cause.
食べすぎた (“ate too much”) emphasizes the idea of overeating.
The original 食べて太りました is a bit more concise and neutral about how much you ate.

Is ばっかり an acceptable alternative to ばかり, and does it change the nuance?

Yes. ばっかり is the colloquial, spoken version of ばかり.

甘い物ばっかり食べて太りました。
It sounds more casual and conversational but carries the same meaning.

Can 私 be dropped completely from this sentence? If so, does it change anything?

Absolutely. Japanese often omits pronouns when context is clear:
最近甘い物ばかり食べて太りました。

The meaning stays the same, but dropping 私は makes it more concise and natural in casual speech. Including 私は just adds emphasis on the speaker.