Learn Hardcore Greek: Βάζω το τηλέφωνο στην τσάντα μου όταν πάω στο γραφείο. - I put my phone in my bag when I go to the office.

In this sentence it expresses a habitual action: something you regularly do whenever you go to the office. Greek uses the simple present for habits, just like English does in I put the phone in my bag when I go to the office.

There is also another related form, βάλω (aorist stem), which you would see after να or θα (for example να βάλω, θα βάλω) to talk about a single, completed act. Here, because we’re talking about a general routine, βάζω (present) is the natural choice.

Why do we say το τηλέφωνο and not just τηλέφωνο?

Greek uses the definite article (here το) much more often than English.

το τηλέφωνο literally = the phone
In this context it means my (or our) phone, the one that is understood from the situation.

In Greek, when you talk about specific, known things (your phone, your bag, the office you work at), you normally use the definite article:

το τηλέφωνο = the (known) phone
η τσάντα = the (known) bag
το γραφείο = the (known) office / my workplace

So Βάζω το τηλέφωνο… is the natural everyday way to say I put (my) phone….

What does στην mean, and why isn’t it σε την?

Στην is the contracted form of σε + την:

σε = in, into, to, at (very general preposition)
την = the (feminine singular, accusative)

So:

σε + την τσάντα → στην τσάντα

Greek almost always contracts σε + article in speech and writing:

σε + το → στο
σε + τον → στον
σε + την → στην
σε + τα → στα
σε + τις → στις

So στην τσάντα means in/into the bag.

Why is it στην τσάντα but στο γραφείο?

The difference comes from the gender of the noun:

η τσάντα (bag) is feminine → article την → σε + την → στην τσάντα
το γραφείο (office) is neuter → article το → σε + το → στο γραφείο

So:

στην τσάντα = to/in the bag
στο γραφείο = to/at the office

Which case are την τσάντα and το γραφείο in, and why?

Both are in the accusative case, which in Modern Greek is used for:

The direct object of a verb.
The object of many prepositions, including σε.

In this sentence:

την τσάντα (μου) is the object of the preposition σε (inside στην).
το γραφείο is also the object of σε (inside στο).

So they are accusative singular:

η τσάντα → την τσάντα
το γραφείο → το γραφείο (neuter has same form in nominative and accusative)

What exactly does μου mean in στην τσάντα μου?

Μου is the unstressed (clitic) form of εγώ in the genitive case, and here it works as a possessive pronoun:

μου = my

So:

η τσάντα μου = my bag
στην τσάντα μου = in/into my bag

Important points:

It comes after the noun it modifies: η τσάντα μου, not μου η τσάντα (the latter is only used for strong emphasis).
It does not change for gender or number of the noun: το βιβλίο μου, η τσάντα μου, τα κλειδιά μου (my book, my bag, my keys).

Could I say όταν πάω στο γραφείο, βάζω το τηλέφωνο στην τσάντα μου instead? Is the word order flexible?

Yes, that word order is perfectly natural, and in fact many speakers would say it that way.

Greek word order is fairly flexible. In this sentence you have two clauses:

Βάζω το τηλέφωνο στην τσάντα μου
όταν πάω στο γραφείο

You can put the όταν-clause either:

At the end:
Βάζω το τηλέφωνο στην τσάντα μου όταν πάω στο γραφείο.
At the beginning:
Όταν πάω στο γραφείο, βάζω το τηλέφωνο στην τσάντα μου.

Meaning is the same; starting with όταν makes the time condition a bit more prominent.

Why is it πάω and not πηγαίνω? Is there a difference?

Both πάω and πηγαίνω mean I go and are very close in everyday usage.

πάω: slightly shorter, very common in speech.
πηγαίνω: a bit more neutral/formal, but also very common.

In this specific sentence, you could use either:

…όταν πάω στο γραφείο.
…όταν πηγαίνω στο γραφείο.

There can be subtle aspect differences in some contexts (like repeated vs single action), but here, talking about a general routine, both are acceptable and natural.

Is πάω here present tense or something like a subjunctive?

In όταν πάω στο γραφείο, πάω is the present tense, indicative mood, 1st person singular.

You can recognize the subjunctive in Greek because it is normally introduced by να:

να πάω = (that) I go / to go

Here we have όταν πάω (when I go), without να, so it is indicative. It expresses a real, repeated situation in time: whenever I go to the office.

Why don’t we say εγώ βάζω at the beginning? Is εγώ omitted?

The subject pronoun εγώ (I) is usually omitted in Greek, because the verb ending already shows who the subject is:

βάζω = I put
βάζεις = you put
βάζει = he/she/it puts

So Βάζω το τηλέφωνο… on its own clearly means I put the phone….

You would add εγώ only for emphasis or contrast:

Εγώ βάζω το τηλέφωνο στην τσάντα μου, αλλά εσύ το κρατάς στο χέρι σου.
I put the phone in my bag, but you keep it in your hand.

Does στο mean “to” or “in”? Is this motion (into the bag) or location?

The preposition σε (and its contracted forms στο, στην, στον etc.) covers several English meanings: to, in, into, at, on.

You understand whether it means motion or location from the verb and the context:

Βάζω το τηλέφωνο στην τσάντα μου
→ The verb βάζω (put) clearly implies movement into the bag → into my bag.
Το τηλέφωνο είναι στην τσάντα μου
→ The verb είναι (is) describes location → in my bag.

So στην here effectively means into, but Greek doesn’t change the preposition; the verb does the work.

Could I say κινητό instead of τηλέφωνο?

Yes. In everyday speech, το κινητό (literally “the mobile”) is very common for mobile phone / cell phone.

So a very natural, colloquial version would be:

Βάζω το κινητό στην τσάντα μου όταν πάω στο γραφείο.

Both το τηλέφωνο and το κινητό are correct; κινητό is just more specific (it clearly means mobile phone, not e.g. a landline).