Learn Hardcore Russian: Сейчас моё главное хобби — писать маленький блог о путешествиях. - Now my main hobby is writing a small travel blog.

Many short foreign loanwords ending in -и (like такси, кино, меню) are treated as neuter indeclinable nouns.
You see neuter agreement with them:
- интересное хобби (interesting hobby)
- моё хобби (my hobby)

In other words, хобби looks the same in nominative, accusative, genitive, etc. Context and agreeing words show you the case and gender, not endings on хобби itself.

What is the function of the long dash — here? Could we use есть or это instead?

The dash — stands in for the linking verb to be in the present tense. Russian normally omits есть (is/are) in such sentences:

Сейчас моё главное хобби — писать маленький блог…

is literally:

Now my main hobby — to write a small blog…

You could say:

Сейчас моё главное хобби — это писать маленький блог…

This is also natural. Это adds a bit of emphasis, like saying my main hobby is this: writing a small blog…

Using есть here (…хобби есть писать…) would sound unnatural in modern Russian. In the present tense, есть is usually omitted except in special emphatic or contrastive contexts.

Why is the verb писать in the infinitive and imperfective aspect here, not написать?

Two things are going on:

Infinitive as a hobby or activity
Russian often uses the infinitive to name an activity as a general concept:
- Моё хобби — читать. – My hobby is reading.
- Его мечта — путешествовать. – His dream is to travel.
So писать блог functions like the noun ведение блога (blogging).
Imperfective for regular/repeated activity
The imperfective писать describes an ongoing or repeated process (writing as an activity), not a one-time completed result:
- писать блог – to (regularly) write / run a blog
- написать блог would sound like a single completed action, which doesn’t fit the idea of a hobby.

So писать is used because a hobby is a regular, ongoing activity, not a one-time achievement.

Is писать блог a natural collocation in Russian, or is something like вести блог more common?

Both are possible, but they have slightly different nuances:

вести блог – the most typical phrase for to blog / to run a blog
Emphasises managing and maintaining the blog over time (posting, updating, etc.)
писать блог – also understandable and used; emphasises the writing itself, the act of producing posts.

In a sentence about a hobby, many native speakers would choose:

…моё главное хобби — вести маленький блог о путешествиях.

But писать маленький блог is not wrong; it just focuses more on the writing process.

Why is it маленький блог and not маленькое блог?

Again, agreement:

блог is a masculine noun (ending in a consonant).
The adjective must match in gender, number, and case.

Masculine singular nominative/accusative adjective ending is -ый / -ий:

маленький блог (small blog)
новый блог (new blog)

Neuter would be маленькое, but that would be used with neuter nouns like окно:

маленькое окно

So маленький блог is correct because блог is masculine.

Which case is блог in, and how can I tell?

In this sentence, блог is in the accusative singular:

писать (что?) блог – to write (what?) a blog

How to tell:

It answers the direct object question что? (what?), so it’s accusative.
For most inanimate masculine nouns, nominative and accusative singular look the same:
- блог (nom.) – этот блог интересный (this blog is interesting)
- блог (acc.) – я читаю блог (I read a blog)

So the role in the sentence (object of писать) tells you the case.

Why do we say о путешествиях and not о путешествия or о путешествиями?

After the preposition о (about), Russian uses the prepositional case:

о ком? о чём? – about whom? about what?

The noun путешествие (a trip, journey) declines like this:

singular: о путешествии
plural: о путешествиях

In the sentence, the speaker means about travels / about trips in general, so the plural is used:

о путешествиях – about travels / about trips

Forms like о путешествия (nominative plural) or о путешествиями (instrumental plural) are simply the wrong cases after о.

Could we move сейчас to another place in the sentence? Would the meaning change?

Yes, сейчас is fairly flexible:

Сейчас моё главное хобби — писать маленький блог о путешествиях.
Моё главное хобби сейчас — писать маленький блог о путешествиях.
Моё главное хобби — сейчас писать маленький блог о путешествиях. (less common, more contrastive)

Nuances:

At the beginning (Сейчас моё главное хобби…) – neutral; focuses on the current situation in general: Right now, my main hobby is…
In the middle (Моё главное хобби сейчас…) – slightly stronger contrast with the past or future: My main hobby now (as opposed to before) is…
After the dash (… — сейчас писать…) – unusual; it sounds more like emphasizing the timing of the action itself.

The basic meaning is the same, but word order slightly changes what is being emphasized.

What is the difference between главное хобби and just хобби here? Could we say самое любимое хобби?

хобби – just a hobby.
главное хобби – main hobby, the most important or central one among several.
самое любимое хобби – favorite hobby, the one you like the most.

All are possible, with different emphasis:

Сейчас моё хобби — писать маленький блог…
My hobby (the one I’m talking about now) is writing a small blog.
Сейчас моё главное хобби — писать маленький блог…
My main hobby right now (vs other hobbies) is writing a small blog.
Сейчас моё самое любимое хобби — писать маленький блог…
My absolute favorite hobby right now is writing a small blog.

So yes, самое любимое хобби is grammatically fine; it just highlights emotional preference instead of importance.

Why is there no explicit verb for English is (like есть) in the Russian sentence?

In Russian, in the present tense with a simple X = Y structure (both are nouns/adjectives/infinitives), the verb to be is usually omitted:

Он врач. – He (is a) doctor.
Моё хобби — читать. – My hobby (is) reading.

The dash — is used in writing to show this connection clearly, especially with longer phrases on each side.

The full form with есть exists, but in modern Russian:

Он есть врач.
Моё хобби есть читать.

both sound archaic or very unusual in normal speech. So the natural present-tense pattern is simply:

Сейчас моё главное хобби — писать маленький блог…
(no verb needed)

How would this sentence change if I wanted to talk about a past hobby, like Back then, my main hobby was writing a small travel blog?

You need:

A past-time adverb like раньше (before) or тогда (back then).
The past tense of быть – было, agreeing with neuter хобби.

For example:

Раньше моё главное хобби было писать маленький блог о путешествиях. – Before, my main hobby was writing a small travel blog.
Тогда моё главное хобби было писать маленький блог о путешествиях. – Back then, my main hobby was writing a small travel blog.

In the past, Russian normally uses был/была/было/были and does not omit the verb the way it does in the present.